IV. IN SECT A: HEX APOD A. 



467 



plate, the scutellum, while between the hinder wings is a similar 

 postscutellum. In many insects one pair of wings is lacking, the 

 anterior pair being retained in the Diptera, the posterior in the 

 Strepsiptera; these are clearly cases of degeneration. The entire 

 absence of wings may occur from two causes; wings have apparently 

 never been developed in some (primary lack of wings of the 

 Apterygota), while there are others in which we must believe that 

 wings once present have been lost, because nearly related forms 

 bugs, lice, etc. have wings, or because certain individuals (male 

 cockroaches, sexual ants and termites) are winged (figs. 506, 528, 

 529). The prothorax of all recent insects is wingless, but in some 

 of the Archiptera of the coal period wing rudiments occurred on 

 this somite. 



As a result of differences in food the alimentary canal (figs. 490, 

 491) varies greatly. The ectodermal 

 stomodaeum begins with a pharynx, 

 which in the sucking insects is a 

 sucking apparatus with radial mus- 

 cles. The oesophagus, which follows, 

 may be widened to a crop (ingluvies), 

 or it may have a caecal outgrowth 

 which in the butterflies may take the 

 shape of a stalked vesicle (falsely 

 ' sucking stomach'). Also ectoder- 

 mal is the gizzard (km,pv), or pro- 

 ventriculus, the chitinous lining of 

 which is toothed for grinding the 

 food. The true stomach, of ento- 

 dermal origin (m, cd), frequently 

 bears blind sacs or gastric caeca (ap) ; 

 in general it is short and its junction 

 with the hinder ectodermal portion, 

 the proctodeum, is marked by the 

 entrance of the Malpighian tubules 

 (vasa Malpighii, vm). The latter, 

 excretory in fuuction, arise from the 

 proctodeal region. The latter is 

 usually differentiated into a small in- 

 testine and a two-regional (colon and 

 rectum) large intestine. The rectum 

 may have enlargements called rectal 

 glands. True glands, however, occur only at the beginning and 



FIG. 490. Alimentary tract of Card- 

 bus auratus. (From Lang, after 

 Dufour.) av, anal vesicle ; arf, anal 

 gland ; cd, stomach with caeca ; ed^ 

 hind gut; m, ingluvies (crop); fc, 

 head; oe, oesophagus; pv, proven- 

 triculu s (gizzard ): r, rectum; vm, 

 Malpighian tubules. 



