32 THE BIOLOGY OF INSECTS 



The rectum opens at the vent (or anus) on the hindmost 

 segment of the insect's abdomen ; through this the rejected 

 remains of the ingested foodstuffs are passed out. The 

 nature of this excrement varies necessarily with the insect's 

 diet. Its aspect often proclaims the kind of solid food- 

 materials that have been devoured ; the little pellets passed 

 by a leaf-eating caterpillar are green and show the micro- 

 scopical characters of leaf-tissue, while the ** frass " ejected 

 by a wood-eating larva has the aspect of fine sawdust. 

 Curious and interesting is the fact that the Uquid excrement 

 of such sucking insects as greenfly, still containing a pro- 

 portion of available carbohydrate, serves as an acceptable 

 food to many kinds of ants which follow and tend the aphids 

 in order to obtain it. Many insects while in the larval 

 state pass no excrement from the food-canal. In the '' ant- 

 lion " grubs of lace- wing flies the vent is closed, while in 

 the grubs of wasps, bees, and ants there is no outlet from 

 the stomach into the hind-gut until the close of larval life. 

 On the other hand, there are many insects — for example 

 mayflies, silk-moths, and botflies — which in the adult 

 condition take no food at all. Their jaws are reduced and 

 useless and their mouths closed, but the various regions 

 of their food-canals are developed like those of their 

 relations that feed until the end of their Hves. The con- 

 sideration of details as to the immense variety of methods 

 of feeding among insects must, however, be postponed 

 until later chapters deaUng with their general habits and 

 mode of Ufe. 



We have seen that the digested and mostly soluble food- 

 constituents are absorbed by the living cells that form the 

 epithehal Hning of the food-canal. Thence they are 

 conveyed to all other tissues of the body in the circulating 

 fluid well known as the blood. The blood and the circu- 

 latory system therefore next demand our attention. 



In most insects the blood is colourless, consisting of a 

 watery plasma with salts, sugar, proteins, and amino-acids 

 in solution, in which float small cells with distinct nuclei 

 and often a tendency to change periodically their shape, 



