INTRODUCTION 5 



definition of living creatures : ** in which is the breath of 

 Hfe." 



Yet, though insects as a group are typically aerial, they 

 afford many and remarkable adaptations to life in water ; 

 especially noteworthy are those members of the class — 

 dragonflies and gnats for example — which pass the early 

 or preparatory stages of their lives in streams or ponds, 

 emerging into the upper air as they acquire the wings whose 

 full development marks always the attainment by an insect 

 of the adult or perfect state. The various modifications 

 connected with such aquatic modes of life will be considered 

 later. 



In turning from breathing to feeding, we fail to find a 

 distinctive method of action for the class of insects generally. 

 Indeed they are remarkable rather for the great variety 

 displayed in the nature of their food and the means by which 

 they procure it. With regard to the latter question, however, 

 attention may suitably be drawn in this introductory sketch 

 to a structural character common to insects and allied classes 

 of the Arthropoda. This is the modification for feeding pur- 

 poses of certain pairs of limbs that belong to those three or 

 four segments of the body that make up the hinder region of 

 the head (Figs. 2, By C, D ; 6, 7, 8, 9). These Hmbs become 

 wonderfully adapted for testing or tasting, for biting or 

 piercing, for licking up or sucking in the widely different 

 substances — such as the tissues or fluids of live plants and 

 animals, the decaying remains of dead organisms — from 

 which insects of different type draw their food supply. 



One or two features in the digestive system of insects 

 are also worthy of note. The outer skin with its secreted 

 covering of cuticle is pushed inwards in such a way as to 

 form the lining of two extensive tracts situated respectively 

 at the front and hinder ends of the food canal ; these are 

 the ** fore-gut " and *' hind-gut " of the arthropodous 

 digestive tube. The " mid-gut," often comparatively 

 restricted in length, is lined by a sheet of living cells uncoated 

 with cuticle ; this cell-layer or epithelium elaborates the 

 juices with their digestive ferments which act upon the 



