Reproductive biology and strategies of nine meloid beetles from Central Europe (Coleoptera: Meloidae)

The reproductive biology of meloid species from Central Europe is investigated by means of laboratory breeding experiments. They show that the total reproductive potential of meloids, with up to 40,000 eggs, largely exceeds prior data. Furthermore, the number of laid eggs strongly relates to the way by which the triungulins find their host. Clutch size correlates significantly with the size of the beetle, while egg size is independent of this. Both clutch size and egg size decrease with each oviposition. For some species, reproductive data are used to demonstrate how they have adapted to their habitat and to point out existing trade‐offs. Based on the obtained results and on an evaluation of the relevant literature, three types of reproductive strategy can be distinguished within the meloids: (1) very high reproductive rates of open field species with phoretic larvae; (2) average reproductive rates of forest populations of species with phoretic larvae and of species the larvae of which search the nests of their hosts actively; and (3) small to average reproductive rates of those species depositing their clutches in the immediate proximity of their hosts' nests.


Introduction
Meloids, also known as oil beetles or blister beetles, are a globally distributed beetle family with about 2,500 species in approximately 120 genera (Bologna and Pinto 2002). They are characterized by the following traits distinguishing them from all other beetle families (cf. Bologna 1991;Pinto et al. 1996). (1) In most of the taxa the first, very mobile larval instar is a triungulin larva (Beauregard 1890;Bö ving 1924;MacSwain 1956;Selander 1991) that is responsible for the spreading and for finding the source of food. (2) With very few exceptions, the meloid larvae are parasites. Depending on the taxonomic position, the larvae feed on the clutches of common field grasshoppers or on the supplies and larvae of solitary bees (Paoli 1932;MacSwain 1956;Greathead 1962). (3) The larvae develop in a hypermetamorphosis (Fabre 1857(Fabre , 1858. The development includes a quiescent stage habitat binding etc. is lacking almost completely. In what follows, the obtained distinguishing features of the reproductive biology of the studied species are presented, and the similarities and differences between the species' reproductive biology are pointed out and discussed against the backdrop of biology and ecology. Furthermore, it is considered if and how meloids can be classified according to criteria of reproductive biology.
The beetles were collected in the field at the beginning of their activity period at different sites in Germany and Austria (for details see Table I).
Most of the species are very rare in Central Europe. Therefore, the number of investigated specimens of the various species differed considerably.

Rearing methods
The rearing, based on the method described by Selander (1986) but adapted to our own requirements, was carried out as follows: the beetles were kept individually in labelled transparent plastic boxes (20 cm613 cm613 cm) with a slitted lid. Except for M. decorus and S. muralis, the bottom was filled up with 3 cm of moist sand serving as oviposition substrate, which was moistened regularly. Since breeding experiments carried out in 1998 showed that moist sand is not an adequate ovipositing substrate for M. decorus, dry soil was used. For S. muralis, a tube made of emery paper (height 15 cm, diameter 10 cm; granulation K 100) was used as substrate, as the literature has reported oviposition on house walls (e.g. Fabre 1857; Friese 1898) in which mason bees had set up their brood chambers. The beetles were kept under natural day-night light conditions at temperatures between 21 and 25uC during the day and between 15 and 18uC during the night. Because, on the one hand, the mating history of the female beetles was not known, and on the other hand, there is likely to be a correlation between mating and oviposition frequency and egg production, females regularly had the opportunity to mate.
Each beetle was weighed in the morning and in the late afternoon on a Sartorius MC1 laboratory balance (precision¡1 mg).
A good food supply was achieved by feeding a surplus of fresh cleavers (Galium aparine Linné) and flowers of dandelion (Taraxacum officinale Weber) or the lesser celandine (Ranunculus ficaria Linné) for spring species, and with a mixed supply of blades of grass and flowers of dandelion for the autumn species M. rugosus. Specimens of L. vesicatoria were fed with common privet (Ligustrum vulgare Linné). The food for Meloe was replaced every morning and evening and was moistened with tap water from a laundry sprayer. The food for L. vesicatoria was renewed every second day. Faeces were removed daily. Sitaris muralis were not fed, since in previous investigations it was found that this species does not take up any food, except droplets of water (J. Lü ckmann, unpublished data). This finding is in contrast to Bologna (1991) who reported S. muralis as a phytophagous species and who summarized the scarce information on host plants.

Parameters and procedure
The following parameters were examined for each species: N maximum frequency of ovipositions (FO max ); N mean interval between two successive ovipositions (IBO); N mean egg clutch weight (ECW); N mean egg clutch portion relative to the body weight before oviposition (ECP); N mean egg length (EL) and mean egg width (EWI); N mean egg weight (EWE) and mean egg weight per mm pronotum length; N mean egg number per oviposition (EN) and mean egg number per oviposition per mm pronotum length; N mean development time of the larvae until hatch (DT); N maximum total egg number of a species (TEN max ).
The following parameters were examined with successive oviposition for each specimen of those species for which the number of ovipositions exceeded half of the maximum number of ovipositions of a current species: N egg clutch weight per mm pronotum length. N egg weight per mm pronotum length. N egg number per oviposition per mm pronotum length. The elytrae of the Meloe species are much shorter than the abdomen and based on personal observations, until oviposition their abdominal size increases with increasing size of the developing eggs. Therefore, the pronotum length of all investigated species was determined as a measure of body size. Egg clutches were carefully dug out of the soil (except for the eggs of S. muralis, which were deposited on the surface of the emery paper tube), cleaned of sand grains and soil particles, and weighed. Shortly after oviposition, length and width of 30 eggs from each egg clutch was determined using a stereo light microscope (magnification 326) with a measurement eyepiece. This was followed by the determination of the number of eggs. For M. violaceus, M. rufiventris, M. uralensis, M. decorus, and L. vesicatoria, all eggs of an egg clutch were counted. For M. proscarabaeus, M. scabriusculus, and M. rugosus, the egg number was estimated by extrapolation: from each egg clutch a sample of eggs was weighed and counted, the total number of eggs in each clutch being estimated from its total weight. Counting the eggs in water-filled Petri dishes prevented the eggs from being damaged. After the egg surfaces had dried, the eggs (except those of S. muralis, which were left on the tube) were transferred into small, sand-filled plastic tubes (height and diameter: 6.5 cm), approximately 2 cm deep. The eggs of S. muralis were covered by a sticky substance, which made it impossible to separate them without destruction. Therefore the egg numbers of this species were determined by counting the larvae after hatch. All eggs were incubated in a climatic chamber at 20uC. Triungulin hatch from eggs and their emergence on the sand surface (except for S. muralis) was recorded daily. In order to simulate the natural soil conditions to which M. violaceus is exposed, the plastic tubes were transferred into a climatic chamber with a temperature of 12uC after larval hatch but preceding their emergence at the surface. After five months, the tubes were transferred back to the climatic chamber set at 20uC.
All female beetles were dissected to evaluate the presence of chorionized eggs in the ovarioles, such a presence indicating that an additional oviposition would have been possible. The number, length, and width of such eggs was determined as described above. Means and standard deviations were calculated for the following parameters: interval between two ovipositions, egg clutch weight, weight proportion of the egg clutch relative to the body weight before oviposition, egg weight, egg length, egg width, number of eggs per clutch and larval development time from oviposition to larval hatch.

Calculations
Based on all means of the individual beetles, the total average and the respective standard deviation were calculated for each species. For all parameters and each species, the minimum and maximum values were determined.

Statistics
If data followed a normal distribution and the variances were homogeneous, mean values were compared using a unifactorial ANOVA, followed by multiple comparisons with LSD post hoc tests. If assumptions for a unifactorial ANOVA were not met, a Kruskal-Wallis ANOVA was carried out, followed by a Mann-Whitney U test.
The following correlations were analysed: 1. Beetle size (expressed as pronotum length) against the species' mean egg clutch weight, mean egg number, or mean egg weight per female. 2. Oviposition number against egg clutch weight per mm pronotum length, egg number per mm pronotum length or mean egg weight per mm pronotum length; in order to eliminate the influence of females with low oviposition frequency, only females with an oviposition abundance exceeding half of the maximum number of observed ovipositions of a current species were taken into consideration. Such species were considered with samples of at least seven females.
For the relationships in (1) Pearson's product moment correlation and for those in (2) Spearman's rank correlation coefficient were calculated, with r5coefficient of correlation and b5regression coefficient (slope of the regression line).
Results of statistical analysis with an error probability P(0.05 were considered as relevant. The software used to perform statistical analyses was SPSS Windows 10.1.3 (SPSS Inc., Chicago, IL, USA).
Due to regular oviposition by M. proscarabaeus, M. scabriusculus, M. rugosus, M. decorus, Lytta vesicatoria, and Sitaris muralis, it can be assumed that the breeding conditions used were adequate. However, for M. violaceus, M. rufiventris, and M. uralensis no optimal breeding conditions could be established.

Number of studied animals and egg clutches
Out of the 141 individuals used for rearing, 126 oviposited and laid a total of 391 egg clutches. Eleven out of the 15 M. violaceus individuals and two out of the six M. rufiventris individuals did not lay any eggs; by dissecting the ovaries, however, the numbers of eggs and their proportions (see above) were determined. For 19 out of the 34 females from Landau, which were conserved in ethanol and received in 2003, the egg numbers and their proportions were determined (Table II). Thus, on the whole, 423 clutches were used for the analysis of the parameters described as relevant to reproduction. The results are summarized up in Table II.

Oviposition
The following differences concerning oviposition were determined between the various species: N The Meloe species preferred the hours of the night for oviposition, while L. vesicatoria laid its eggs mainly during the day. Thus, for M. proscarabaeus, M. rugosus, and M. decorus, 76%, 53%, and 54% respectively, of ovipositions took place at dusk or night, while for L. vesicatoria, 64% of ovipositions took place during the day.
N Except for M. decorus, the soil tubes were at least as deep as the body length of the ovipositing female, thus completely hiding the animals during oviposition. Meloe decorus, however, dug small hollows with a maximum depth of 1 cm, so usually only the female's  abdomen was hidden. Therefore, the laid eggs were directly deposited under the soil surface and sometimes even visible when looking from above.
N Having completed the oviposition and sealing the tube, the females of the Meloe species and of L. vesicatoria began food intake, while S. muralis did not eat anything during the whole activity period.
N After the sealing of the soil tube, the egg clutches of all of the Meloe species and of L.
vesicatoria were surrounded by the substrate. On the other hand, the clutches of M. rugosus from the fertile plains of the Elbe river near Magdeburg always had a hollow space surrounding them; the hollow space above the clutch was up to 10 mm high, and up to approximately 3 mm wide. However, such a hollow space was lacking for the clutches laid by M. rugosus from the dry location at the Neusiedlersee in Austria.
Frequency of oviposition, interval between two successive ovipositions, number of eggs, and development time of the larvae Between the species, the maximum frequency of oviposition (FO max ) varied quite considerably. While for M. rugosus and M. scabriusculus up to ten or eleven egg clutches per individual were observed respectively, S. muralis only laid eggs once. The dissection of the M. decorus individuals that had oviposited eight times showed that for one of the animals its ovaries were still filled with ripe, chorionized eggs. Thus, we may assume that a ninth oviposition would still have been possible. For the other species that could be analyzed, five to six ovipositions per female were determined. With 9.2 days, L. vesicatoria showed the highest mean interval between two successive ovipositions (IBO); the shortest was determined for M. decorus with 2.5 days. In some cases, females of this species even laid eggs on two successive days. For M. scabriusculus and M. rugosus, the mean interval amounted to 3.6 and 4.6 days respectively, and did not differ statistically.
The mean number of eggs per clutch (EN) varied both between and among species. This became obvious when observing the large standard deviations and the large differences between minima and maxima. This resulted from the fact that the number of eggs depended on the beetle's size (expressed as length of the pronotum, see below). The highest mean number of eggs per clutch was observed in M. proscarabaeus, amounting to 6,194. For some specimens of this species, the number of laid eggs amounted to almost 10,000. The lowest mean number of eggs per clutch amounted to 941 in L. vesicatoria, while in one case a clutch consisted of 72 eggs. The relationship between the species M. proscarabaeus and M. violaceus, which belong to the nominate subgenus was remarkable: in the former species, a clutch on average consisted of more than three times as many eggs as in the latter.
With almost 40,000 eggs, the highest maximum total number of eggs (TEN max ) was determined in M. proscarabaeus and M. scabriusculus, S. muralis exhibited the lowest number, with 2660 eggs.
The mean development time of the larvae till the hatch (DT) was longest in M. violaceus, amounting to 124.2 days, and thus exceeded 10-fold that of M. decorus, which with a development time of 12.1 days, had the fastest developing eggs among the studied species. When compared with M. proscarabaeus, the larvae of M. violaceus needed about six times as long until they hatched. Apart from that, differences between species of the same subgenus were small. Thus, species of the subgenus Eurymeloe on average needed 18.5 (M. rugosus) and 22.8 (M. scabriusculus) days, those of the subgenus Micromeloe between 12.1 (M. decorus) and 15.6 (M. uralensis) days. Within the species, the development time varied very little. The standard deviation never exceeded 2 days.
After the hatch, while sclerotizing, the Meloe and Lytta larvae remained for 2-3 days in the substrate, before emerging at the surface. By contrast, the larvae of M. violaceus left the substrate after having been kept for 5 months at approximately 12uC and subsequently having been kept at 20uC. The larvae of S. muralis remained motionless in a clump under the layer of empty eggshells.

Characteristic features of the eggs and its proportions
The eggs of the surveyed species were oval, one of the ends being wider and having a blunter rounding off than the other. The eggs of all of the Meloe species were between dark and light orange, those belonging to L. vesicatoria and S. muralis had a whitish colour. In contrast to the other species, the eggs of S. muralis were covered by a transparent, oily and sticky substance, which was almost insoluble in water and had the eggs sticking strongly together. At the narrower end of the egg, the substance shaped into a drop that appeared to be part of the egg. Even after the hatch of the larvae, the empty eggshells stuck together, offering shelter to the larvae, which remained motionless under them (see above).
Depending on the species, the eggs were between 0.5 and 2.0 mm long, the width being approximately one-third of the length. The mean egg length (EL) and width (EWI) of the various species in almost all cases significantly differed from one another, respectively (Table II). On average, M. violaceus laid the largest eggs. These eggs were almost three times as long and more than twice as wide as those from M. rugosus, which laid the smallest eggs. Furthermore, the difference between the mean egg proportions of M. violaceus and M. proscarabaeus deserves a particular mention: the eggs of the former species were approximately one and a half times longer and wider than those of the latter.

Egg weights
Also between the mean egg clutch weights (ECW) of the studied species, large differences were found. Meloe violaceus and M. proscarabaeus laid the heaviest egg clutches, with weights of 605.4 and 565.8 mg, respectively, while M. rugosus laid the lightest, weighing 46.9 mg. The mean clutch weights of the species belonging to the subgenus Eurymeloe differed significantly from one another (LSD test: mean difference514.2378, P,0.05).
In order to be able to consider the reproductive effort independently of the individual's size, the proportion of the clutch weight relative to the body weight preceding oviposition was determined (ECP). This value was highest for S. muralis, with 57.2% (in one case it reached 62.5%), for M. scabriusculus, M. rugosus, and M. decorus it was lowest with about 18.6%. Within any subgenus, this proportion remained roughly invariable, amounting to 39.9% in the subgenus Meloe, and varying between 18.6 and 18.7% in the subgenus Eurymeloe, and between 18.5 and 20.9% in Micromeloe. The reproductive effort of M. rufiventris and L. vesicatoria was approximately the same, with 31.9 and 27.2% respectively.
The relationships found for the egg weights (EWE) resemble the ones found for the egg proportions. With 253.3 mg, M. violaceus laid the heaviest eggs and M. rugosus, with 19.2 mg, the lightest. The eggs laid by M. violaceus on average were almost 2.7 times as heavy as those of the closely related species M. proscarabeus (LSD test: mean difference5160.7814, P,0.001), those of M. scabriusculus were significantly heavier than those of M. rugosus (LSD test: mean difference514.2378, P,0.05). Those laid by M. decorus and M. uralensis did not differ significantly (LSD test: mean difference56.0590, P.0.05).

Correlation between parameters relevant to reproduction and the animal's size
For all species that were analyzed, a positive, statistically significant relation between the mean clutch weight (RBCW) or, respectively, the mean clutch size (RBEN) and the beetle's size-expressed as length of the pronotum-was given (Table III; Figures 1, 2). Nevertheless, the egg weight (RBEW) was shown to be independent of the body size for most species. An exception was M. proscarabaeus (Pearson correlation: r50.491, P50.003), for which a significant relation was observable.

Correlation between parameters relevant to reproduction in the course of the reproductive period
Since this analysis required a minimum sample size of seven animals, it could only be carried out for M. proscarabaeus, M. rugosus, M. decorus, and L. vesicatoria (Table IV). For all of these species, the number of eggs (EN) and the egg weight (EWE) decreased with each oviposition, consequently so did the clutch weight (ECW). With the exception of M. rugosus, the decrease in egg weight was significant in all of the considered species. For the parameter clutch weight, this reduction was significant for M. rugosus, M. decorus, and L. vesicatoria, for the parameter egg number it was significant for M. rugosus and L. vesicatoria.

Reproductive capacity
As shown by the results described here, the reproductive potential of meloids is even larger than previously assumed. Not only the frequency of oviposition, but also the mean and Table III. Female size and parameter correlation. Correlation between beetle size (expressed as length of the pronotum) and, respectively, one of the following parameters: mean egg clutch weight per female (RBCW), mean egg number per female (RBEN), and mean egg weight per female (RBEW). maximum clutch size as well as the total reproductive rate of the surveyed species exceed, in some cases considerably, the correspondent available values of other species (a summary of the data about the reproductive biology, as found in the available literature, can be found in Table V). Up to now, in the genus Meloe, the maximum frequency of reported ovipositions amounted to six in M. dianella Pinto and Selander (Pinto and Selander 1970), whereas in this survey 11 ovipositions were observed in M. scabriusculus. Higher frequencies of oviposition are only known for species of the Epicauta vittata group, with a maximum of 22 ovipositions (Adams and Selander 1979). With up to 10,000 eggs per clutch, M. proscarabaeus largely exceeds the former maximum number of 6,572 eggs reported for M. menoko Kono (Kifune et al. 1973). Due to the dependence between the mean egg number per clutch and the size of the females (Figure 1; Table III; note that size depends on the amount of food available for the larvae in the bee nests), between the individuals of a species a strong variation in clutch size results (Table II); this is illustrated by the often large standard deviation.
With 25,175 eggs for M. rugosus and almost 40,000 eggs for M. scabriusculus and M. proscarabaeus, their total reproductive performances also considerably surpass the highest numbers formerly observed in meloids, with 17,200 eggs for M. menoko (Kifune et al. 1973). Very large clutches were also observed in Cissites auriculata Champion. Bianchi (1962) estimated that a clutch he had found in the field comprised about 22,800 eggs. However, he stresses that the clutch should very likely belong to several animals that had successively added to it. When considering data about reproductive performance, one must always take into account that the individual total egg number (as well as the number of ovipositions) will obviously always depend on the females' life expectancy and on the interval between two ovipositions. Therefore, it is to be expected that for some of the species, the data supplied by the literature will be subject to some corrections.
In some cases, the egg-laying performances of the meloids surpass those of other beetle and non-social insect species many times over. Depending on the considered taxonomic group, between one and several dozen (compare Table V) (e.g. species of the genera Eletica, Hycleus, Linsleya, Epicauta, and Gnathium), several hundred (e.g. species of the genera Pyrota, Pseudopyrota, Lytta, Meloe, and Zonitis), or a few thousand eggs (e.g. species of the genera Wagneronota, Berberomeloe, Cissites, and Meloe) are laid. Figure 2. Correlation between mean egg weight per oviposition and female and the beetle's size (expressed as pronotum length), determined separately for each species (*not applicable for Meloe rufiventris, since three out of four females had the same pronotum length). Table IV. Modification of the standardized reproduction parameters with each further oviposition. Correlation between oviposition number and, respectively, one of the following parameters: egg clutch weight (ECW) per mm pronotum length, egg number (EN) per mm pronotum length, and egg weight (EWE) per mm pronotum length, calculated for females with oviposition abundance exceeding one half of the maximum observed number of ovipositions of the respective species. For the carabids Carabus granulatus Linné and Pterostichus quadrifoveolatus Letzner, Thiele (1977) mentions mean total egg numbers that amount to 41 and 136, respectively. According to Klausnitzer (2002), e.g. Melolontha hippocastani Linné lays a total of 60-80 eggs, Dytiscus marginalis Linné up to 1000, and Leptinotarsa decemlineata (Say) up to 2,500. The clutches of the females belonging to butterfly Lymantria dispar Linné comprise approximately 800 eggs (Dettner and Peters 1999). For the dragonfly Pyrrhosoma nymphula Sulzer, Corbet (1999) estimates the possible total number of laid eggs at about 8,500.

Reproduction investments
Usually, within a given species, large individuals produce more offspring (Fox and Czesak 2000). This relationship was observed in all of the studied species (Figure 1), confirming also the results found for the species of the E. vittata group (Adams and Selander 1979). Except for M. proscarabaeus, for which a significant positive correlation between egg weight and animal size was found (Table III; Figure 2), the egg weight was independent of the ovipositing female's size. This could have a positive effect on the larvae, since the larger offspring of many insects (note that egg weight here is used synonymously to offspring size) grow faster, are more robust in the face of environmental influences, become larger, and their developing juveniles also have a higher survival probability (Fox and Czesak 2000).
During the reproductive period, the egg number and weight decreased with each oviposition for M. proscarabaeus, M. rugosus, M. decorus, and L. vesicatoria. For the egg weight of M. proscarabaeus, M. decorus, and L. vesicatoria this relationship was significant, for the egg number a significance was found in M. rugosus and L. vesicatoria (Table IV). Thus, these results are in contrast with the results of Adams and Selander (1979), who for the species of the E. vittata group reported a constancy of clutch sizes over time (changes in egg volume were not considered). Therefore, larvae, as for M. proscarabeus, may not only profit from having larger mothers (see above), but also from hatching from eggs that have been laid sooner in the reproductive period. However, it is unclear if the reduction in the larvae's size, as observed with each oviposition, is somehow biologically relevant.

Trade-offs
Nevertheless, larger species do not necessarily produce more offspring. On the contrary, a female's reproductive resources are always limited, thus leading to a negative correlation between number and size of the offspring. Therefore, the division of the available resources between number and size of the offspring is a characteristic compromise for each species and has to be interpreted as an adaptation of the life cycles to the respective living conditions (Stearns 2000). The reproductive effort, having been determined as the proportion of the clutch weight relative to the body weight preceding the oviposition, does not vary statistically within any one Meloe subgenus. However, the division of resources into number and size of the offspring can in some cases differ quite strongly. Thus, while M. proscarabaeus invests mainly into the number of offspring, M. violaceus invests mostly into their size.
A different relationship exists between M. proscarabaeus and M. scabriusculus and M. rugosus. The lower mean clutch size of the two latter species is compensated for with a higher ovipositioning frequency, thus levelling out the total reproductive output of the three species.
No differences in the reproductive biology were found between M. decorus and M. uralensis. Exhibiting a similar reproductive effort, both species on average lay similar numbers of similarly weighing eggs. When compared with M. scabriusculus and M. rugosus, however, they invest their similar relative reproductive effort mainly into offspring size, while M. scabriusculus and M. rugosus invest into their number. The different trade-offs in the two groups of species are probably due to differences in their host-finding strategies. Since the phoretic host-finding of M. rugosus and M. scabriusculus implies that their larvae have a reduced probability to reach a suitable host, by comparison to those of M. decorus and M. uralensis, a compensation with high numbers of offspring is mandatory. On the other hand, it is understandable that M. decorus and M. uralensis generate larger and thus probably fitter larvae, since their hosts are actively searched for (see below).

How to find a host and risk of mortality
The high reproductive rates of the Meloe species are probably due to high larval losses. The triungulins of the Meloe species, while waiting on flowers for a suitable host, attach themselves to all hairy insects visiting these flowers (Fabre 1858;Precht 1940), resulting in larvae being repeatedly found e.g. on honeybees, dipterans, coleopterans, and lepidopterans (Cros 1931;Lü ckmann and Kuhlmann 1997). However, Harrington (1895) (quoted by Pinto and Selander 1970) showed that the larvae are at least partially capable of choosing. Though the larvae of an unknown Meloe species were repeatedly found on Lasioglossum discus (Smith) (5Halitctus discus Smith), Prosopis modestus Say (5P. affinis Cresson), and Ceratina dupla Say, they were not observed on honeybees or Andrena nivalis Smith, also frequent in the area. A similar observation is reported by Pinto and Selander (1970) for larvae of M. angusticollis Say. This leads to the assumption that the phenomenon of phoresy on insects other than hosts is probably not fortuitous, but could represent a dispersion event from the hatching point, as a specialization to reduce the competition for hosts and increase the possibility to find an available bee nest (Pinto and Selander 1970), but also connected with a high larval lost due to unsuccessful larvae.
However, under given circumstances, phoresy is also likely to be subject to relatively low larval losses. Species of the genus Stenoria and Hornia as well as Apalus bimaculatus (Linné), S. muralis, and C. auriculata pursue brood care by laying their clutches into the entrances of their hosts' nests or in the immediate proximity of their hosts (Stenoria spp., A. bimaculatus, S. muralis, C. auriculata) or alternatively, by laying them in the cells where they hatched (Hornia spp.).
Besides the species of the Lyttini, Pyrotini, Cerocomini, Epicautini, and Mylabrini, also M. decorus and M. uralensis appear to actively reach their hosts' nests. Thus, studies have shown (J. Lü ckmann, unpublished data) that triungulins belonging to these species, having been placed on or at the side of dead bees, quickly left the bees or avoided them, instead of attaching to their hairs, as for example, observed in our experiments for M. proscarabaeus, M. violaceus, M. scabriusculus, or M. rugosus. Alternatively, on the ground the larvae exhibit a distinct searching behaviour, as observed for the larvae of L. vesicatoria. This hypothesis is supported by Vrabec et al. (2002), who during their studies never found larvae of M. decorus on bees which had been captured in colonies in which the females had laid their eggs. Nevertheless, both host-finding strategies may be used, since G. Schumann (unpublished data) provides reports of triungulins of M. decorus (det. Lü ckmann) that were observed on flowers of Potentilla verna Linné. In order to shed some light on this problem, further studies are planned.
Differing from that of the other Meloe species, the host-finding behaviour of M. decorus and M. uralensis would also explain the fact that when compared with M. scabriusculus and M. rugosus, they exhibit different trade-offs concerning the mean clutch size and the size of the offspring (5egg weight; Table II). Larger offspring may have a higher fitness and higher survival probability than smaller offspring (review in Fox and Czesak 2000). That is why it is beneficial for M. decorus and M. uralensis to invest more into each offspring, while M. scabriusculus and M. rugosus produce a larger number of offspring. A compromise favouring the size of the offspring, as found for M. decorus and M. uralensis, can also be observed for the non-phoretic larvae of L. vesicatoria.

Reproduction and habitat selection
The significance arising from the resemblances and differences in the parameters relevant for reproduction can only be understood in the light of the species' ecology and habitats (Stearns 2000). Thus they should be considered as an adaptation of the meloids to the phenology of their hosts as well as to the peculiarities typical of their habitat. For the surveyed species, only in a few cases does knowledge about the habitat requirements of populations from Central Europe exist, which might be different from populations existing in for example, Southern Europe. In what follows, the presented results will be discussed keeping this knowledge in mind.

Meloe violaceus and Meloe proscarabaeus
The species M. violaceus and M. proscarabaeus have roughly the same geographic distribution (Bologna 1991). In Central Europe the adults of both species have the same activity period and can be found between approximately mid-March and the beginning of June. But there is a great difference in the larval phenology. Whereas in Central Europe the larvae of M. proscarabeus can be found between mid-May and mid-July, those of M. violaceus are active between mid-April and the beginning of May (J. Lü ckmann, in preparation), i.e. at the same time as the imagines (also cf. Havelka 1980). In M. violaceus, the overlapping of the activity periods of adults and larvae on the one hand, and the prolonged development time and the stay in the soil during the next months on the other hand, suggests that the larvae found during the spring have hatched from eggs laid during the previous spring (also cf. Pinto and Selander 1970).
The differences in the activity periods of the larvae of M. proscarabaeus and M. violaceus is founded in the populated habitats. While in the plains of Central Europe M. proscarabaeus populates open and warm habitats, M. violaceus is a stenotopic species in the plains which populates river hardwood forests. For the latter, only during the brief time window of early spring, thanks to the sparse foliage of the trees, is the forest still light and the light intensity on the forest floor reaches its maximum (Hofmeister 1997). This short period, during which the early-flowering plants, e.g. Anemone nemorosa Linné and Ranunculus ficaria Linné, blossom, is the only time of the year during which the mostly light-and warmthloving solitary bees can use the forest as a feeding and nesting habitat (Westrich 1990), and during which the triungulins have the only chance to meet a potential host.
Both species invest the same amount of biomass into their offspring. However, while M. proscarabaeus invests this amount into many, smaller offspring, M. violaceus divides it between fewer but larger eggs. The differences in the reproductive biology of these two species can be explained by the different length of stay of eggs or larvae in the soil and by their phenology. The fact that the eggs or larvae of M. violaceus remain in the soil for almost a year and that the fitness and survival probability of larger offspring is higher than those of smaller ones (Fox and Czesak 2000) is probably why fewer but larger offspring are produced by M. violaceus.

Meloe rugosus
Primarily, M. rugosus from Central European populations is a river meadow species that appears to find optimum conditions in the direct flooding area, on sandy, lightly elevated, moderately grassy areas that are sparsely covered with shrubs and trees. Secondarily, however, it can also be found, for example, in sand, gravel and clay pits as well as on arid grasslands. Basically, M. rugosus is an autumn species with its main activity period in October and November (own data; Vrabec and Hess 2001;also cf. Vrabec 2003), and sometimes also being found in the following months (own data; .
Its characteristic reproductive biology should be considered an adaptation to the dynamic conditions in the river meadows. Thus, compared with other Meloe species, the females lay their eggs in short intervals, by means of which good and possibly only short lasting weather conditions are efficiently made use of. The relatively small mean clutch size, as compared with M. proscarabaeus, is compensated for with a larger ovipositioning frequency. The hollow space, which is up to 10 mm high above the clutch and approximately 3 mm wide to its side, surrounds the clutch and is characteristic of animals inhabiting river meadows, whereas it is lacking in animals from dry locations. Filled with air, it probably serves the purpose of allowing a quick seeping away of the water after a flooding.

Meloe decorus
Meloe decorus is a typical spring species, active from mid-March to about the end of April. Quite characteristic for the reproductive biology of M. decorus, with a time interval of 2.5 days, their females, among all known Meloe species, exhibit the shortest time interval between two ovipositions (also cf. Vrabec 1993), the eggs being laid next to the soil surface and developing very quickly (cf. Vrabec et al. 2002). According to our own observations, most of the time the eggs are laid directly into the wild bee colonies (also cf. Vrabec et al. 2002). All of this can be interpreted as an adaptation to the early and short activity period. As for M. rugosus, favourable weather conditions can be made the best possible use of thanks to short intervals between the ovipositions. The low depths at which the clutches are laid make it possible for the intense solar radiation to heat up the upper soil layer fast enough, thus accelerating the hatching of the larvae and shortening the development time. By ovipositing within the bee colonies and the ability to enter the host's nest actively, the probability of finding a suitable host is relatively high. This explains why M. decorus manages with drastically lower egg numbers than, for example, M. proscarabaeus and M. rugosus.

Sitaris muralis
Sitaris muralis is a typical summer species, the larvae of which spend the time after the hatch, lasting from late summer till next spring, under the layer of dried eggshells (Fabre 1858;Friese 1898;Cros 1910). Its reproductive biology in Central Europe is characterized by the females ovipositing only once, the clutch comprising a mean number of 1,700 eggs and the clutch weight amounting to 57.2% of the body weight, more than for any other of the surveyed species. Since the adults do not take up food during their entire lifespan, the resources for oogenesis are obtained by the complete reduction of the flight muscles (J. Lü ckmann, in preparation), a phenomenon similar to the oogenesis-flight syndrome (van Huizen 1977;Nelemans 1987;Rankin et al. 1994). The egg clutches are then laid directly into the passages of the mason bees or deposited in their proximity. Due to the vicinity between the clutch or larvae and their potential hosts, the probability of attaching to a bee is relatively high, which is why this species presumably only invests into a relatively low number of eggs, the total number of eggs being very low by comparison with all other species.

Reproductive strategies within meloid beetles
Based on the obtained results and the evaluation of the existing published reproduction data, which are summarized in Table V violaceus from Central European populations, the probability of finding a host is considerably increased compared with the first group. The risk of larval losses is compensated for with average reproductive rates. N Group 3: here belong C. auriculata (Horiini), S. muralis, Tricrania sanguinipennis (Say) as well as species of the genera Apalus, Allendeselazaria, Glasunovia, Ctenopus, Sitarobrachys, Stenoria, Nyadatus, and Hornia (all Nemognathini). Although their larvae are phoretic, the females depose their clutches into the entrances of their hosts' nests or into their immediate proximity (Stenoria spp., A. bimaculatus, S. muralis, C. auriculata) or, alternatively, into the cells in which they themselves hatched (Hornia spp.). This way, the probability of finding a host is relatively high. Therefore, larval losses are made up for either with small to average clutch sizes and low ovipositioning frequencies (e.g. Hornia spp., C. auriculata) or else with larger individual clutches (e.g. A. bimaculatus, S. muralis).
clutches, comprising a few dozen or a few hundred eggs, on the flowers, young shoots, or leaves of their host plants, larvae phoretically reaching their hosts' nests. Since Erickson and Werner (1974b) stress that the females of the Nemognathinae in most cases only lay eggs once, in rare cases also twice, further studies should clarify which reproductive strategy is being pursued by the representatives of these groups. Possibly, the results of Werner (1974a, 1974b) and of many other authors are, however, based on suboptimum rearing conditions, which implies that with better suited methods one would obtain higher ovipositioning frequencies and therefore higher total egg numbers. For Meloe species with relatively low clutch sizes, for example, M. campanicollis Pinto and Selander or M. dianella Pinto and Selander, it may be guessed that, due to the specific nature of the habitat, the egg numbers were reduced as compared to other species (cf. M. violaceus from Central Europe), but that they are compensated for with high ovipositioning frequencies and/or that the larvae are not phoretic.
When referring to life and reproductive strategies it occurs over and over again that the concept of r-and K-strategy (MacArthur and Wilson 1967;Pianka 1970) is discussed. Although a series of examples exists, for which the r/K-pattern appears to fit (Begon et al. 1991), it is not capable of explaining all realized life cycles, therefore a generalization of the assumption of such dichotomous life strategies appears to be poorly suited (Stearns 1977(Stearns , 1992. The surveyed meloids, too, cannot be integrated into this concept, since the species are characterized both by r-and K-selecting features, thus making a straightforward classification into one of the two types quite impossible (Table VI).