292 



NATURAL HISTORY. 



Fig. 11. STIIUCTUKE OF EYE OF COCK- 

 CHAFER. 



but which is really the outer termination of a nervous rod springing from the surface of the 

 expanded end of the optic nerve. The nervous rods and conical bodies ai-e enveloped by a 



layer of pigment separating them from their fellows. Thus, 

 each of these thousands of facets may be regarded as possessing 

 the structure of a distinct eye (Fig. 11). 



The antennae appear to be the only other organs of sense 

 possessed by insects, but it is exceedingly difficult to ascertain 

 what sense it is their special function to serve, if, indeed, they 

 may not have different offices to perform in different insects. 

 They have been supposed to be organs of hearing and of smell, 

 the former partly on observational grounds, and partly by analogy 

 with the Crustacea ; but while we may be certain that insects 

 possess these senses, it is very difficult to point out their seat. 

 The antennae, however, in some cases, are certainly tactile organs. 

 Special organs of hearing have been described in particular 

 insects by various authors. In the common Grasshoppers they 

 have been supposed to be placed in the sides of the abdomen, 



in those of another family at the base of the anterior tibiae. The faculty of hearing has also 

 been assigned to the hind wings of Beetles, and to the halteres of the two-winged flies. The 

 possession of this sense by insects may perhaps safely be predicated from the fact that many of 

 them have the faculty of producing sounds, generally by the friction of the wings, or of the 

 legs against the wings, but in some cases by the agency of special organs. 



The digestive organs (Fig. 12) commence by a pharynx attached to the organs of the mouth, 

 and sometimes produced into free processes (kypopharytix and epipharynx), which especially in 

 sucking insects may take part in the formation of the mouth. This narrows into a gullet or 

 cesophayus, which runs through the anterior segments of the body, and becomes widened behind 

 into a first stomach or crop. In many sucking insects, this dilatation of the oasophagus is not 

 in the direct course of th3 alimentary canal, but placed on one side and united with the 

 oesophagus by a narrow canal ; it is then known as the sucking stoniac/u The abdominal part 

 of the intestinal canal, presents great differences in different insects and groups of insects. In 

 general in vegetable-feeders the intestine is comparatively simple in its character, but of considerable 

 length and much convoluted ; in carnivorous forms, on the other hand, it is shorter and runs more 

 directly to the anal orifice, but is generally divided into several distinct sections, which have 

 received special names. Thus, in many cases we find a gizzard (prorentriculus) a short, more 

 or less spherical, strongly muscular part, the inside of which is often furnished with several 

 horny (chitinous) ridges ; and beyond this a much longer and broader stomach, of delicate texture, 

 with no chitinous lining, but commonly with a glandular layer, which often gives the surface 

 of the stomach a villous appearance. Beyond this true stomach comes the intestine proper, which 

 often presents a clear division into different regions ; and at the point where the stomach and intestine 

 join certain long, slender, blind tubes, known as the Matpiyhian vessels, usually open. These 

 were at one time supposed to represent the liver, but they are now regarded as analogous to the 

 kidneys in the higher animals. Besides these glandular organs we find in the anterior part 

 of the body one or two pairs of salivary glands, which are also blind tubes, sometimes extending 

 back within the abdomen, and not unfrequently possessing a reservoir in the neighbourhood of 

 the mouth. Their secretion is discharged into the mouth during the mastication of the food. 



Other glands not connected with the alimentary canal need only a passing notice. Odoriferous 

 glands ai*e not uncommon. They may be situated in various parts of the body, and have their 

 orifices situated in the soft skin uniting the segments or at the joints of the limbs ; or in the 

 neighbourhood of the anal orifice, where they produce an acid secretion, which collects in a small 

 vesicle, from which it is ejected as a means of defence. Silk glands are of common occurrence in 

 the larvae of insects. They consist of a pair of long, blind tubes, placed one on each side of 

 the abdominal region, and communicating by a long duct with an orifice in the labium. The 

 secretion of the3o glan.ls has the property of hardening into a fino thread when exposed to the 



