page_title: Morphology of Diptera
section_title: Head
hierarchical_section_title: Morphology of Diptera / Adult / Head
context_page_description: The Diptera is a very large and diverse order of mostly small to medium-sized insects. They have prominent compound eyes on a mobile head, and one pair of functional, membraneous wings, which are attached to a complex mesothorax. The second pair of wings, on the metathorax, are reduced to halteres. The order's fundamental peculiarity is its remarkable specialization in terms of wing shape and the morpho-anatomical adaptation of the thorax – features which lend particular agility to its flying forms. The filiform, stylate or aristate antennae correlate with the Nematocera, Brachycera and Cyclorrhapha taxa respectively. It displays substantial morphological uniformity in lower taxa, especially at the level of genus or species. The configuration of integumental bristles is of fundamental importance in their taxonomy, as is wing venation. It displays a complete metamorphosis, or holometabolous development. The larvae are legless, and have head capsules with mandibulate mouthparts in the Nematocera. The larvae of "higher flies" are however headless and wormlike, and display only three instars. Pupae are obtect in the Nematocera, or coarcate in Brachycera.
context_section_description: {{More citations needed section|date=August 2020}
  The head is distinct from the thorax, with a marked narrowing at the neck. In "lower flies" (Nematocera), it is prognathous (head horizontally oriented with the mouth anterior), in "higher flies" (Brachycera) it is hypognathous (head vertically oriented with the mouth ventrad).
  The shape of the cranial capsule also varies. In the Nematocera, the dorsal–ventral part of the head extends forward from the eyes due to the development in length of the clypeus and subgenal area (subgena), the distal end of the extension is the 'mouthparts'.
  In the "higher" Diptera the head has a subglobose shape and the fronto-clypeus is an area bounded superiorly by the eyes and the vertex. In Cyclorrhapha Schizophora, a morphological element of particular importance is the presence of the ptilinal suture formed by the resorption of the ptilinum after emergence from the pupa.The suture separates two regions: 1. the upper one is the frontal region, which has continuity with the apex, the orbital region and the gena, 2. the lower one, the face or  clypeus, contains the insertion of the antennae and ends with the epistomal edge which comprises the upper lip.
  The eyes are usually very obvious, but reach a remarkable development in the Brachycera. In this suborder the eyes are markedly convex and have grown to occupy most of the side of the head. The space between the two eyes can sometimes be reduced to a narrow strip running from the front of the occipital region, or disappear altogether because of the direct contact between the eyes or their margins. The morphology of the compound eye is characterized by a significant number of ommatidia, of the order of thousands in muscoids. The ocelli, when present, are located in the top of the head, arranged at the corners of a triangle in an area called stemmaticum or ocellar triangle.
  For the purpose of systematics, the presence, the arrangement, and the conformation of the cephalic bristles is important and they have a specific terminology. Bristles on the head are: frontal bristles, sometimes named lower orbital bristles are located on 
  the frontal plates of the frons resembling a small alley extending from the base of the antennae toward the vertex and edging the median frontal stripe laterally. Sometimes they are situated lower, along the frontalia below the antennal attachment and over a greater or lesser distance. Orbital bristles are located on the vertex plates of the frons and usually restricted to its upper half. They may be arranged in longitudinal rows named inner and outerorbital bristles. (The frontal and vertex plates of the frons can be visualised on the basis of the arrangement of the frontal and orbital bristles); ocellar bristles are located on the vertex between the ocelli; outer and inner vertical bristles are located on the border between the vertex and the occiput and near the upper corner of the eyes; postvertical bristles are located behind the ocelli on the occiput near the median line of the head; vibrissae usually arrayed in small numbers along the facial sections of the arcuate suture, near the margin of the oral cavity; sometimes they ascend along the suture over a greater or lesser distance, occasionally almost to the place of antennal attachment; false vibrissae-bristles are placed along the margin of the oral cavity. 
  Sometimes the terminology is conflicting. For instance in the Acalyptratae there are usually two bristles, more or less strong, positioned along the posterior margin of the ocellar triangle. These bristles are called "postvertical bristles" in old literature, since the nineteenth century, and the term is used sometimes in the recent literature. Steyskal (1976) proposed the name "postocellar bristle" the adopted term in the Manual of Nearctic Diptera (McAlpine, J.F., 1981) and the Manual of Palaearctic Diptera (Bernhard Merz, Jean-Paul Haenni, 2000) and, therefore, this term occurs widely in the literature that refers to these two fundamental works. Two other bristles, present only in some families of Acalyptra
caption: Head morphology of a muscoid dipteran (schematic representation)1: labellum; 2: lower lip (labium), 3: maxillary palp, 4: upper lip (labrum); 5: subgenal area; 6: clypeus; 7: fronto-orbital area; 8: fronto-orbital bristles; 9: outer vertical bristle; 10: inner vertical bristle; 11: postocellar bristles (postvertical bristles in old literature); 12: ocelli; 13: ocellar bristles; 14: compound eye; 15: Frontal suture or ptilinal suture; 16: antenna; 17: arista; 18: vibrissa.