108 AN INTRODUCTION TO ENTOMOLOGY 



the longitudinal axis of the body, as is represented in the diagram 

 given above (Fig. 105). In most insects, however, it is longer than 

 the body, and is consequently more or less convoluted (Fig. 124); 

 great variations exist in the length of the alimentary canal as com- 

 pared to the length of the body; it is longer in herbivorous insects 

 than it is in those that are carnivorous. 



The principal divisions. Three chief divisions of the alimentary 

 canal are recognized; these are termed the fore-intestine, the- mid- 

 intestine, and the hind-intestine, respectively. In the embryological 

 development of the alimentary canal, the fore-intestine and the hind- 

 intestine each arises as an imagination of the ectoderm, the germ 

 layer from which the hypodermis of the body-wall is derived (p. 29). 

 The imagination at the anterior end of the body, which develops 

 into the fore-intestine, is termed the stomodceum; that at the posterior 

 end, which develops into the hind-intestine, the proctod&um. Between 

 these two deep invaginations of the outer germ layer of the embryo, 

 the stomodaeum and the proctodasum, and ultimately connecting 

 them, there is developed an entodermal tube, the mesenteron, which 

 becomes the mid-intestine. 



These embryological facts are briefly stated here merely to 

 elucidate two important features of the alimentary canal: first, the 

 fore-intestine and the hind-intestine are invaginations of the body 

 wall and consequently resemble it in structure, the chitinous lining of 

 these two parts of the alimentary canal is directly continuous with 

 the cuticula of the body wall, and the epithelium of these two parts 

 and the hypodermis are also directly continuous; and second, the 

 striking differences, pointed out later, in the structure of the mid- 

 intestine from that of the fore- and hind-intestines are not surprising 

 when the differences in origin are considered. 



Imperf orate intestines in the larvae of certain insects. In the larvae 

 of certain insects the lumen of the alimentary canal is not a continuous 

 passage; in these larvae, while food passes freely from the fore- 

 intestine to the mid-intestine, there is no passage of the waste from 

 the mid-intestine to the hind-intestine ; there being a construction at 

 the point where the mid-intestine and hind-intestine join, which 

 closes the passage during a part or the whole of the larval life. This 

 condition has been observed in the following families: 



(a) Hymenoptera. Proctotrypidae (in the first larval instar), 

 Ichneumonidae, Formicidae, Vespidae, and Apidae. 



(b) Diptera. Hippoboscidae. 



