Chap, iv.] ENTER ON OF INSECTS. 133 



soldiers, but reduced in the workers ; the Ephe- 

 meridse or day-flies, which want to eat no food in the 

 adult stage, have the gnathites almost completely 

 aborted. The Mallophoga, or so-called " Mandibulate 

 lice," which are found on the skin of birds and mam- 

 mals, and feed on their feathers and hairs, have the 

 mandibles hooked and the maxillae small. 



Like the crayfish, the cockroach has a large 

 portion of the anterior region of its digestive tract 

 lined with chitin, and, like that form, it has also a 

 considerable portion of the hinder region formed by 

 the proctodeal invagination. The chitinous layer 

 extends through the funnel-shaped pharynx, the 

 narrower 03sophagus, the crop-like enlargement, and 

 the proventriculus ; the last has the form of a trun- 

 cated cone, and its walls are thick and well-provided 

 with muscles; its internal lining is raised up into 

 ridges which serve as teeth, and between these ridges 

 there are pouches. The next succeeding portion has 

 no chitinous lining, and its anterior end has connected 

 with it eight blind prolongations (the so-called pyloric 

 caeca), which are not all of the same length, and 

 which vary in size according to the periods of digestive 

 activity ; it is, apparently, in this cavity that the 

 food undergoes the changes which convert it into 

 . chyle, and the caeca are only to be regarded as out- 

 growths which increase the capacity of the ventriculus. 

 The intestine behind is lined throughout with chitin, 

 and the smaller is separated from the wider portion 

 by a valve ; the whole tract ends in a terminal anus. 



As may well be supposed, the different parts of 

 the digestive tract present very different characters 

 in the various groups of insects in the mandi- 

 bulate forms (Neuroptera, Coleoptera) the stomach 

 is provided with a series of more or less powerful 

 chitinous ridges, by means of which the food is 

 comminuted; in the sucking insects the gizzard 



