CIRCULATORY APPARATUS OF INSECTS. 



293 



air, and by means of it the larva? are enabled to form protective coverings for themselves, and 

 especially to prepare the cocoons in which so many of them pass their pupal period of existence. 



A considerable portion of the interior of the body of an insect is occupied by a peculiar fatty 

 substance called the adipose body, which is especially abundant in the full-grown larvae, and 

 consists of a yellowish, lobulated mass lining the walls of the body-cavity, and filling up the 

 spaces between the viscera. It would appear to be a 

 store of nutriment to be used up in the final maturation 

 of the insects, as it often diminishes in volume in pro- 

 portion as the reproductive organs are developed. 



The circulatory apparatus of insects is sufficiently 

 simple. It consists of a sort of vessel running along the 

 dorsal part of the insect, and divided by constrictions into 

 a series of chambers corresponding in number with the 

 segments of the abdomen, and then continued forward in 

 the form of a simple tubular vessel (aorta), through the 

 thorax to the head. The chambered portion part of this 

 dorsal vessel is attached to the walls of the abdomen by a 

 series of triangular muscles, which spring in pairs from 

 a broad base on each side of each chamber, becoming 

 narrowed towards the place of their attachment to the skin 



of the abdomen. The blood within the dorsal vessel is 



driven forward by the successive contractions of the 



chambers until it is forced out from the anterior orifice of 



the aorta, whence it returns through the interspaces of the 



various organs (lacunie) to the abdomen again. It then 



passes into the sort of sinus formed around the dorsal vessel 



by its muscles above described, and thence into the vessel 



itself through a series of valvular openings between the 



successive chambers, to be again driven out by the contrac- 

 tions of the organ. There are consequently neither arteries 



nor veins in the insect-body, and the circulation of the 



blood is strictly what is called lacunar. 



Respiration is effected by means of an immense number 



of branched tubular organs called trachea, which com- 

 municate with the outer air by a series of peculiar apertures 



in the integument, known as the stiymata. The latter are 



situated on each side of the body, in the boundaries 



between the successive segments, but the head is altogether 



destitute of them, and the last pair of abdominal stigmata 



are frequently wanting. In the abdomen they are 



frequently placed in the membrane uniting the dorsal and 



ventral plates. In form the stigmata are sometimes round, 



when they are enclosed by a horny ring and furnished Fig. 12. DiGrsrivE APPARATUS OF DYTICUS. 



internally with bristles or hairs, converging towards the "> <E801)hasus: c - Giz ^L < ! l j- g o'im""ti > ne. : e> Smali Inte9tinc; 



centre to prevent the entrance of injurious particles. In 



other cases they constitute more or less elongated slits, and are then capable of being closed by a pair 



of lip like parts, which also frequently bear spines or bristles, sometimes branched or pectinated. The 



closure of these stigmata is often effected by the agency of small chitinous pieces embedded in the 



membranous parts, which show a singular resemblance to the little bones found in the mammalian ear. 

 The traclwce, into which these apertures admit the air, are tubular organs, branching through all 



parts of the body, and gradually diminishing in diameter towards their final ramifications. In this 



way they convey the air to all the organs, which they, at the same time, bind together and hold in 



position, thus, to a considerable extent, performing the double office of respiratory organs and 



