588 ZOOLOGY SECT. 



3. GENERAL ORGANISATION. 



The exoskeleton of the Insecta (Fig. 477) consists of a chitinous 

 cuticle (cut.}, which varies in hardness and thickness in different In- 

 sects, and in different parts of the body of the same Insect, but is very 

 rarely calcined. Frequently it presents hexagonal markings ; some- 

 times it is perforated by numerous pores ; sometimes it is covered 

 with thin scales ; in many cases it is' developed into tactile hairs or 

 setce, which may be scattered over the 

 body, or may be located only on certain of 

 the appendages the antennae, the maxil- 

 lary and labial palpi, and the tarsi of the 

 legs. In some glands are present in the 

 integument odoriferous, honey-secreting, 

 or wax-formrhg glands ; poison glands are 

 present in connection with an abdominal 

 sting in certain insects ; spinning glands, 

 forming a silky material, are confined to 



base, basement membrane ; the 

 cut. layers of the cuticle ; _.. 



epi. ' epidermis ; set. seta. I he head presents no , trace of seg- 



mentation, but the history of its develop- 

 ment indicates that it may be looked upon 



as composed of a prostomium and about five segments, intimately 

 united together. It varies a good deal in shape, but always presents 

 the regions that have already been described in the case of the 

 Cockroach. Sometimes the head is sunk within the anterior part of 

 the thorax ; sometimes it is free from the latter ; and there may be, 

 as in the Cockroach, a short narrow region or neck, covered with 

 soft skin, supported only by isolated cervical sclerites, intervening 

 between the two on the ventral aspect. 



The three segments of the thorax -pro-, meso-, and meta-thorax 

 are usually firmly united together ; but in some Insects the pro- 

 thorax is movable upon the other segments : it is usually the 

 smallest of the three segments. In each the exoskeleton consists 

 of dorsal or tergal and ventral or sternal elements, sometimes 

 separate from one another laterally, sometimes united together in 

 such a way as to form complete rings round the segments. Laterally 

 projecting processes or pleura are sometimes developed. 



The abdomen contains from seven to eleven segments, enclosed 

 in tergal and sternal shields. In some Insects the first abdominal 

 segment is united with the thorax so as to appear to belong to 

 the latter region. 



The appendages of the head are four pairs, as in the Cock- 

 roach ; but a considerable variation is observable in the different 

 orders, especially as regards the jaws. In a few eyes are absent. 

 Most have large compound or faceted eyes, and many have simple 

 eyes or ocelli as well ; in a few groups the latter are alone present. 



