INSECTS. 



[ 433 ] 



INSECTS. 



n 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 halteres of the Diptera and the elytra 

 of beetles present in certain parts a multi- 

 tude of vesicular projections of the external 

 membrane ; and a nervous filament passes 

 to each. Hicks considers them to be organs 

 of smelling. 



The abdomen (fig. 361 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 last segment of the abdomen 

 of many insects in the females, is furnished 

 with five or six valves or setae, some of 

 which form stings, saws, or borers, 

 according to the function they perform as 

 weapons of defence, or to cut through the 

 tissues of plants or animals, so that the eggs 

 may be deposited in them. The saws are 

 well-known interesting objects. See CHAL- 

 CIDID^E, CYXIPID^E, and STINGS. The ab- 

 domen contains the principal 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 oeso- 

 phagus (PI. 35. fig. 2 6), 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 (fig. 2 c), 

 which is distinguished by the frequently 

 great development of its lining membrane 

 into plates, teeth, or hooks of horny tissue 

 (PL 34. 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 (c?), in which digestion takes place. 

 4. Behind this is a longer or shorter small 

 intestine (PL 35. 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 homogeneousperitoneal layer j 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 six peculiar organs of doubtful 

 nature, arranged in pairs, either transversely 

 or longitudinally. These consist of trans- 

 parent rounded, oval, or elongated tubercles, 

 projecting inside the colon, sometimes with 

 a horny ring at the base, and traversed by 

 tufts o'f tracheae. These organs are most 

 numerous in the Lepidoptera. 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 duct?, 

 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 

 many in&ects, caecal appendages arise from 

 the latter, and also contain cells which se- 

 crete a biliary liquid. 



In some insects the small intestine is fur- 

 nished with glandular appendages in the 

 form of tubular caeca, probably representing 

 a pancreas. 



Intimately connected with the digestive 

 and assimilative process is a curious organ 

 called the fatty body. This attains its maxi- 

 mum of development towards the end of the 

 larval period of existence. It consists of a 

 number of fat- cells imbedded in a reticular 

 or lamellar tissue (PL 35. fig. 28), composed 



