394 Introduction to Animal Morphology. 



arrangement of the stigmata, which may be two, 

 sometimes tubular, posterior (in aquatic larvae), or four 

 (two in front on the second somite, as well as two be- 

 hind, as in headless Dipteran larvae), but usually they 

 are more numerous. In adult insects they are in the 

 soft intermetameric rings under cover of the wings. 

 There is one pair in aquatic Hemiptera (Nepa). 

 Smynthurus has one pair of stigmata on the head be- 

 neath the antennae. Papirius has no tracheae, nor has 

 the larva of Chloeon for its first few stages before its 

 tracheal gills develop. Podura has four pair of abdo- 

 minal stigmata; others have even one on every post- 

 cephalic somite. The stigmata may have a raised 

 border (Peritreme), valves, bristles or a sieve like 

 cover (CEstrus larvae, Lamellicornes). These tracheae 

 may be simple unbranched, or may have simple anas- 

 tomosing tubular branches, or may have vesicular di- 

 latations, with no spiral thread, as in some flying 

 insects. The anastomoses may produce continuous 

 longitudinal or transverse canals. 



Many insects emit sounds which are audible to us, 

 and probably many others produce notes too high for 

 our appreciation. They may depend on the rubbing 

 of the body rings together, the streaming of air from 

 the stigmata, the rubbing of the wing covers against 

 the back or the hind legs, or the head against a chiti- 

 nous plate. 



All insects are normally oviparous and dioecious, 

 the sexes often differing conspicuously. The testes 

 are two, rarely twelve, in a pair of clusters, as in some 

 beetles, branched or pouch-like, united into one in 

 some Lepidoptera, in a connective sheath [tunica 

 vaginalis). From each passes a vas deferens, which 



