6 THE BIOLOGY OF INSECTS 



food (Figs. 10, ii). Such action may go on in the fore-gut 

 or hind-gut also, by the transference thither of portions of 

 food mingled with the juices, and the digested food-material 

 may be absorbed through the thin cuticle lining those 

 regions of the canal, and so traverse their wall, as well as 

 the wall of the mid-gut, to pass into the blood for the 

 general nourishment of the body. 



In most animals that have attained to a high or even 

 moderate degree of organisation there is a circulating fluid 

 or blood which serves as the medium of exchange between 

 the general living tissues of the body and the organs of 

 specialised function, bringing to the tissues food and oxygen, 

 providing the raw material for their secretions, and taking 

 up from them waste-substances to be carried to and 

 eliminated by the organs of excretion. As a rule these 

 exchanges are carried on while the blood flows through 

 vessels minutely fine with exceedingly thin walls which 

 afford opportunity for diffusion in either direction. But 

 in insects, as among Arthropoda generally, the blood during 

 much of its circulatory course flows through great spaces 

 in the body, surrounding the digestive and other organs 

 and bathing them on all sides ; thus instead of a typical 

 body-cavity (coelom) avast blood-space (haemocoel) occupies 

 most of an insect's inside. From this principal cavity where 

 food-material is absorbed through the gut-wall, the blood 

 passes up through a perforated membrane into a special 

 blood-space — relatively long and broad but shallow — just 

 beneath the dorsal body-wall where the narrow tubular 

 heart is placed, gaining entrance to the heart by means of a 

 series of paired slits (Fig. 12, h). The heart's rhythmical 

 pulsations force the blood towards the front region of the 

 insect where it passes from the system of closed tubes into 

 the great blood-space already described. 



The elimination of nitrogenous w^aste-matters — among 

 the most important end-products of the chemical changes 

 (metabolism) always going on in a living body — is performed 

 in insects by a set of organs highly characteristic of the class. 

 These are elongate, narrow tubes (Fig. 10, Mt) which grow 



