TYPE TRACHEATA. 



All 



traction of certain dorso-ventral muscles of the abdomen, 

 which cause a compression of the organs in that region of the 

 body, inspiration following on their relaxation and the conse- 

 quent re-expansion of the abdomen. In structure the tracheae 

 are simply to be regarded as invaginations of the body-wall, 

 and consist of a single layer of cells continuous with the hypo- 

 dermis of the body, lined within 

 — that is to say, on the surface 

 with which the air is in contact 

 — with chitin, which is thick- 

 ened in such a way as to form 

 a spiral band extending along 

 the tube and serving as a spring 

 to keep its walls apart. 



The digestive tract is in 

 most groups a straight tube, 

 but in Insects (Fig. 227) it may 

 be coiled in a more or less com- 

 plex manner and differentiated 

 into several parts. Glands of 

 various kinds are usually asso- 

 ciated with it, salivary glands 

 (Fig. 227, sg) opening into the 

 anterior portion and Malpighian 

 tubules {mv), in connection with 

 the posterior portion, being 

 the most constant in occur- 

 rence. It is to be noted that 

 the fore-gut and hind-gut are 

 ectodermal in origin, and that 

 the Malpighian bodies arising 

 as outgrowths from the hind- 

 gut are also ectodermal, differing 

 thus in origin from the similarly- 

 named organs of the Arachnida, which are apparently of 

 endodermal origin, arising from the mid-gut. In function 

 both organs are similar, the Malpighian bodies of Tracheates 

 being excretory. 



The nervous system in the less-differentiated members of 



Fig. 215. — Figure showing thb 



DlSTKIBUTION OF TeACHE^ IN 



Aphis 'pelargonii (after Witlaczil). 

 At = antennse. 

 g = gland-duct. 

 St = stigma. 

 tr = trachea. 

 2, 3 = Ihoiacic appendages. 



1, 



