TYPE TRACHEATA. 



471 



At 



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 baud 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 Malpighiau 

 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 Malpighiau 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 THE 

 DISTRIBUTION OF TRACHEAE IN 



Aphis pelargonii (after WITLACZIL). 



At = autennse. 

 g = gland-duct. 

 st = stigma. 

 tr trachea. 

 1, 2, 3 = thoracic appendages. 



