The Interactive Fly

Genes involved in tissue and organ development

Thorax and Abdomen



The thorax consists of three segments: prothorax, mesothorax and metathorax moving from anterior to posterior. Each segment possesses a pair of legs. The wings attach to the mesothorax. The haltere, a pair of small paddle-like organs used to maintain balance in the adult fly, attach to the metathorax.

The prothorax is greatly reduced in Drosophila compared with other insects and serves as a support for prothoracic legs. Its dorsal portion or notum, is a narrow collar extending across the thorax, almost concealed by the mesonotum of the next segment.

All three thoracic segments have fused to form an almost solid box. The mesothorax has enlarged to enable this segment to support the entire flight system including the wings and the internal musculature that drives flight. Viewed anterior to posterior, one first finds the notum. The next structure, posterior to it and occupying the greater part of the mesonotum called the scutum of the mesothorax. Further on, just behind the wing bases sits the shield like scutellum, and beyond that the postnotum. The ventral pleural elements of the mesothorax are divided into posterior and anterior regions. These regions lie below the legs (subcoxal). The wing bases show axillary sclerites (hardened areas). Each leg is divided into six segments: coxal, trochanter, femur, tibia, metatarsis and tarsus (from proximal to distal). The halteres on the metathorax are the morphological equivalent of the metathoracic wings.


The abdomen contains seven pairs of holes (spiracles), one pair in each segment. The eighth and last segment of the female contains the anus and no spiracles. In the female, the vulva lies between the apices of the gonopods. These irregular tubes extend into the body from the surface. Two gonopods terminate internally and are known as spermatheca. Males have a genital opening between their ninth and tenth abdominal segments. The spiracle of the seventh segment has been moved forward to a more anterior position in the segment. The ninth segment is highly modified and bears a large dorsal plate that extends to become a narrow band running beneath the anus. The tenth segment is a pair of sclerotized plates flanking the anus.

Genes affecting thoracic and abdominal development

The dorsal ventral patterning of the thorax is regulated by the dorsal-ventral polarity system and segmentational patterning regulated by gap, pair rule and segment polarity genes. Thoracic identity is determined by teashirt and modified by two genes of the Antennapedia complex (Sex combs reduced and Antennapedia), and by bithorax complex gene Ultrabithorax. The two transcripts of Antennapedia take on different roles in ectoderm and endoderm and in the three thoracic segments. Ultrabithorax expression spans the last thoracic and first abdominal segments.

How is the very sharp division between adult thoracic and abdominal identity regulated? It should be kept in mind that the adult thorax is derived from the wing imaginal disc and leg discs, while the adult abdomen has a completely different embryological origin, being derived from nests of histoblasts held in the embryonic and larval epidermis.

Seven gene systems are involved in the most posterior adult segments. The terminal system (torso), regulating tailless is required for the posterior segments. Tailless regulates the posterior stripes of even skipped and fushi tarazu, which in turn regulate the segment polarity genes. The second gene system regulating tail morphogenesis is the posterior group genes of the terminal system. Inhibition of Hunchback translation by Nanos allows for the expression of caudal, important for the last two abdominal segments and the telson. The dorsal ventral polarity of the abdomen is regulated by the dorsal system and the segmental division of the abdomen is initiated by gap genes, carried out by pair rule genes and refined by segment polarity genes. Finally, segmental differences are regulated by the bithorax complex. Specifically, the regulatory transcript of Abdominal B is important in determining the identity of the posterior most tail segments. Independent regulation of abdominal A in each of the abdominal segments by the infra-abdominal regions of the bithorax complex, determine segment identity in the abdomen. Ultrabithorax is expressed at the juncture of thorax and abdomen, spanning the third thoracic segment and the first abdominal segment.


Ferris, G. F. (1950). External Morphology of the Adult, pp. 368-419, In: The Biology of Drosophila. M. Demerec, editor. Wiley: New York

date revised: 9 may 96 
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