INSECTS. 



[ 359 ] 



INSECTS. 



two elytra are rubbed across each other, the 

 bow being drawn across the ridges gives 

 rise to the peculiar sound, the intensity of 

 which is increased by the tympanum acting 

 as a sounding-board. The apparatus of the 

 grasshopper is essentially of the same struc- 

 ture. It must be stated, however, that va- 

 rious other explanations of the origin of the 

 stridulating noise produced by these insects 

 have been given. Thus by some authors the 

 two bows are stated to work across each 

 other, whilst by others the legs are supposed 

 to act against the bow. This subject pos- 

 sesses interest for future observation. 



In other insects, there is a peculiar mecha- 

 nism for uniting the anterior and posterior 

 wings of each side, so that they may be kept 

 steady and may act in unison during flight. 

 In the Lepidoptera, the moths only are pro- 

 vided with a minute hook arising from the 

 base of the costal nerve of the lower wing, 

 and inserted into a socket near the base of 

 the main nerve, on the under side of the 

 upper wing. In the Hymenoptera, there are 

 many such hooks arranged along part of the 

 costal nerve at the anterior and upper 

 margin of the second pair of wings (PI. 2/. 

 fig. 13). When the wings are expanded, 

 these attach themselves to a little fold on 

 the posterior margin of the anterior wing 

 (fig. 11 ri), along which they play freely 

 when the wings are in motion, sliding to and 

 fro like the rings on the rod of a window- 

 curtain. These hooks are somewhat twisted 

 towards their free end, recurved and some- 

 times notched at the point. They vary in 

 number in different genera and even in 

 the sexes. In the Hemiptera the whole 

 margin of part of the anterior wing is hooked 

 over a corresponding recurved part of the 

 posterior, so as to produce the same effect. 



The abdomen (fig. 365 ri) forms the third 

 and terminal portion of the body of insects. 

 It usually consists of nine or ten rings or 

 joints, the posterior of which, however, are 

 sometimes so concealed, so small or so fused 

 with the others, that they appear to be 

 absent. The abdomen contains the prin- 

 cipal part of the alimentary canal and its 

 appendages, with the organs of reproduction. 



The alimentary canal varies in length in 

 different insects, and even in the same insect 

 at various periods of its development. It 

 consists of the following parts : 1 . The ceso- 

 phagus (PI. 28. fig. 2 b), a muscular organ 

 extending through the thorax ; it is some- 

 times dilated to form a crop or ingluvies, as 

 in the Lepidoptera, Hymenoptera, and Di- 



ptera, and this occasionally forms a lateral 

 sac, connected with the oesophagus by a 

 narrower portion only, and called a sucking 

 stomach. 2. Next follows the muscular 

 stomach, proventriculus or gizzard (PL 28 c), 

 which is distinguished by the frequently 

 great development of its lining membrane 

 into plates, teeth or hooks of horny tissue 

 (PL 27. fig. 1) ; these serve to triturate the 

 food, and have long been known as beautiful 

 microscopic objects. 3. This is succeeded by 

 a long, cylindrical, true stomach or ventri- 

 culus (d), in which digestion takes place. 

 4. Behind this is a longer or shorter small 

 intestine (PL 28. fig. 2, between d and/), 

 terminating in 5, a dilated portion, forming 

 a large intestine or colon ; behind which is 

 a short rectum. The structure and length 

 of the parts of the alimentary canal vary 

 generally according to the nature of the 

 food, although this is not always the case in 

 regard to the latter. 



The alimentary canal is covered by an 

 outer homogeneous peritoneal layer; beneath 

 which is a muscular coat, consisting of lon- 

 gitudinal and transverse fibres. Internally 

 it is lined by a homogeneous epithelial layer, 

 consisting, in part at least, of chitine. Be- 

 tween the latter and the muscular coat, at 

 the middle of the alimentary canal, is a layer 

 of cells, which probably perform a glandular 

 function. The large intestine or colon of most 

 insects in the imago state contains from four to 

 sixpeculiarorgansofdoubtfulnature,arranged 

 in pairs, either transversely or longitudinally. 

 These consist of transparent, rounded, oval 

 or elongated tubercles, projecting inside the 

 colon, sometimes with a horny ring at the 

 base, and traversed by tufts of tracheae. 

 These organs are most numerous in the Le- 

 pidoptera. They are never found in insects 

 in the larva- or pupa-state. 



In most insects salivary glands are present 

 as one, two, or rarely three pairs of colour- 

 less sacs or tubes of very variable form and 

 length, sometimes scarcely extending beyond 

 the prothorax, at others accompanying the 

 alimentary canal into the abdomen. They 

 consist of an outer homogeneous envelope, 

 lined with colourless, nucleated cells, and 

 frequently have one or more distinct ducts, 

 sometimes containing a spiral fibre; they 

 terminate near the mouth, in some insects 

 the ducts previously expanding into a reser- 

 voir. 



A distinct liver is not present in insects, its 

 function being performed by the glandular 

 cells in the walls of the true stomach. In 



