530 THE ANIMAL PARASITES OF MAX 



We need only observe the following characters in considering the anatomy of 

 insects : 



The EPIDERMIS consists of the chitinous cuticle, which is separate from the 

 cellular layer beneath (hypodermis) ; the various appendages are supported by the 

 chitinous layer. 



The INTESTINAL CANAL usually consists of the anterior, median and terminal 

 intestine, and as a rule passes straight through the body ; salivary glands discharge 

 into the anterior part, and, in some cases, yield a stiffening secretion which serves 

 for spinning webs ; numerous or scanty hepatic tubes are appended to the median 

 intestine, while on the border between the median and terminal intestine open four 

 to six long tubes (vasa malpighiana), which act as urinary organs. Finally the end 

 portion of the intestine carries various glands (anal and rectal glands, etc.). 



The CENTRAL NERVOUS SYSTEM agrees in structure with that of the Annelids, 

 but is more highly developed. The pharyngeal ring surrounds the front part of the 

 intestine ; the sensory nerves originate from its SUPERIOR PHARYNGEAL GANGLIA 

 and are the seat of the higher psychical functions; the INFERIOR PHARYNGEAL 

 GANGLIA govern the mouth parts, and in addition appear to regulate the movements 

 (cerebellum) of the vertebrates. 



The chain of GANGLIA lying on the ventral side of the abdomen consists primi- 

 tively of pairs of ganglia corresponding with the twelve segments, which are connected 

 by longitudinal and transverse commissures. But many changes in the ganglia may 

 be seen in insects caused by partial or entire amalgamation of single ganglia, so that 

 in a few cases only one abdominal ganglion is present. In conclusion, a definite 

 INTESTINAL NERVOUS SYSTEM is always present. 



Of the organs of sense the FACETTED EYES, situated at the sides of the head, 

 deserve special mention, as do also the ORGANS OF TOUCH and SMELL, situated on 

 the antennae, and the ORGANS OF HEARING and taste, or finer sensations, situated at 

 the mouth and in the buccal cavity. 



The sounds emitted by insects are, as a rule, produced by the friction or beating 

 of certain chitinous parts, but sounds are also produced in breathing (flies). 



The ORGANS OF RESPIRATION, the so-called trachea?, are highly developed ; there 

 are openings (stigmata) at the sides of the body which draw in air by means of the 

 active participation of the muscles of the body. The number of stigmata varies 

 between two and ten pairs ; the tracheae themselves branch off from the trunks in 

 the most varied manner, and carry air to the internal organs. 



The colourless BLOOD circulates between the tissues and organs, and is kept 

 circulating by the contraction of a chambered dorsal vessel provided with ostia, 

 and which terminates in a short aorta opening at the anterior end. 



Insects are SEXUALLY DISTINCT ; their sexual glands are in pairs and have a 

 tubular structure, but the testicular tubules are united together by a capsule into an 

 oval testicle ; exceptionally, also, the excretory canals are double, as also the sexual 

 orifices ; usually the paired canals unite into a single oviduct or spermatic duct 

 which terminates at the posterior end of the body after receiving the products of 

 various glands. 



As to the HISTORY OF THE DEVELOPMENT of insects, all that is necessary to 

 mention here is that the young hatched from eggs only exceptionally (as in 

 Apterygpta) resemble the adult parent (insecta ametabola) ; as a rule they differ 

 from them not only in the shape of the body, but also more or less by their manner 

 of life, and only attain the form of the parent through METAMORPHOSIS. This is a 

 gradual process (insecta hemimetabola) in the Rhynchota and Orthoptera, or a 

 sudden one with a stage of inanition (insecta metabola) in the other orders. This 

 stage of rest or inanition, the PUPA, concludes the larval life (caterpillar, maggot, 



