378 



THE AGRICULTURAL NEWS. 



November 27, 1909. 



INSECT NOTES. 



NATURAL HISTORY OF INSECTS. 

 Part III. Growth (concluded), Sense.s, Cikculatiox. 



The body of a caterpillar, which may be considered 

 a typical insect in the larval condition, is composed of 

 thirteen segments, as follows : — the head, which appears as 

 one; the thora.x-, three; and the abdomen, nine. The segments 

 of the thorax and abdomen are chitinous rings, joiniMl 

 together by means of a flexible connective tissue, which 

 allows for great freedom of movement, and, as already stated, 

 for a certain amount of growth in size. In most adult 

 insects, it is not easy to distinguish the segments which go 

 to make up the thorax and abdomen, because these are often 

 much modified in accordance with the habits or structure of 

 the insect. The growth of insects always takes place during 

 the larval portion of the life-cycle. Caterpillars, for instance, 

 moult, or .shed their skin, four or five times during their 

 growth, from hatching to pupation. Adult insects do not grow 

 in size. Once the wings have been developed, the insect is full 

 grown. Small Hies do not grow into big flies, small moths do 

 rot grow into large ones, nor small beetles into large beetles. 

 It should be remembered that butterflies and moths, 

 beetles, flies, bees and wasps all develop from a larval stage 

 which is very different in appearance from the adult form, 

 and that the change in appearance is brought about in the 

 period of pupation. 

 On the otlier hand, 

 such insects a s 

 grasshoppers, cock- 

 roaches and cotton 

 stainers, w h i c h 

 Lave an incomplete 

 metamorphosLs, are 

 somewhat like the 

 adult in general 

 form, but without 

 wings or w i t h 

 developing win^s. 



Fk;. 45. LoNc.iTUDiNAL Section through Caterpillar. 



In the case of any insect, however, the 

 winged individual is the adult. 



SENSES. 



Insects have well developed sensss of taste, smell, 

 hearing and sight. They have also well developed digestive, 

 nervous, circulatory, and respiratory systems. 



It is next to impossible to determine exactly the 

 functions of certain sense organs in insects. The organs for 

 seeing, hearing and for producing sound can be recognized 

 by their structures, and by experimetit, but there are others 

 (if which it is not so easy to determine the function 



The sense of taste is probably located in the small 

 appendages of the month parts, the senses of smell anil hear- 

 ing are, in some insects at least, in the antennae, which are 

 often the most consiiicuous ap|)cndagcs of the head. Special 

 organ.s of hearing are sometimes (in certain Orthoptera) to 

 be found on the abdomen and legs. The sense of sight 

 depends on the eyes, which are of two kinds, simple and 

 compound. The compound eyes ate often made up of many 

 facets and are located at the sides of the head, while the 

 simple eyes, or ocelli, are situated between them, either on the 

 top or the front of the head. 



The sin'pic eyes of insects are not always present, but 

 when they occur, they are two or three in number. The 

 compound eyes are perhaps the most comj)lex and delicate 

 structure in the insect world. 



In many instances, the compound eyes occupy by far the 



greater part of the entire surface of the head, and the number 

 of elements or 'facets' which go to make up one of these 

 compound eyes often runs to many thousands, although some- 

 times it is small. 



It is not likely that insects have jiower of virion over 

 long range, or that they .see distinctly. They are all conscious 

 of light and darkness, and some of them distinguish certain 

 colours; mauy of them distinguish moving bodies, and others, 

 perhaji.?, recognize at greater or less distances those insects on 

 which they prey, or insects or birds to which they are likely 

 to J'all a (iiey. 



§;,-. CIRCULATION. 



■i-^-f.-;- The arrangement of the circulatory, digestive and 

 ■ nfervous systems in insects is a typical characteristic. The 

 alimentary canal is central in position; the organ of circulation, 

 is dorsal, and on account of its position, has received the 

 name of d<irsal vessel; the nervous system is ventral. The 

 diagram (Fig. 4.5) represents a median longitudinal section- 

 through the body of a caterpillar. The dorsal vessel is repre- 

 sented by the dark line, the digestive section (A L) by the 

 cro;.s-shaded portion, and the nervous system by the dotted 

 lines. The blood of insects is not confined in veins, arteries and 



capillaries, as is 

 the case in the 

 higher animals; 

 it fills the body- 

 cavity and 

 bathes all the 

 organs, even 

 penetrating into 

 the legs and 

 wings. It is 

 usually colour- 

 less, butis.some- 

 red, but does not 

 and it is not 

 blood of the higher 



times inted yellowish, green, and even 

 get its red colour from red corpuscles 

 red blood in the same way as is 

 animals. 



The organ of circulation is the heart or dorsal vessel. It is, 

 as has been said, called the dorsal vessel because of its position 

 in the body, lying as it does along the median line of the back, 

 just under the body wall. The heart is a straight, unbranched, 

 tubular organ provided with muscles extending from the 

 hind end of the body to the head; it is generally closed at the 

 posterior end an<l open at the anterior one. The portion lying 

 within the abdomen is con.stricted at intervals, and at each 

 constriction there is a valve. These valves divide the dorsal 

 vessel into compartments or chamberH. l>y means of 

 a rythmic contraction and cxpan^ion of the walls of the 

 dor.sal vessel, accompanied by a regular opening and closing 

 of the valves, the blood is forced from the rear toward 

 the head, to the long aorta-like portion of the dorsal 

 ves.sel lying within the thorax. The chambers are provided 

 with small openings which communicate directly with the 

 body cavity, and through which the blood in the body cavity 

 is taken into the heart and put into circulation. 



The pulsations of the dorsal vessel and the movement of 

 the blood in the body cavity can easily be observed in certain 

 caterpillars The arrowroot worm, or canna leaf roller 

 (Cii/jioih.t et/i/iiif), is the best of our common West Indiaa 

 insects for the purpose. 



