92 ENTOMOLOGY 



sensations of insects are necessarily inferences from our own sensory ex- 

 periences, they are inevitably inadequate. While it is certain that in- 

 sects have at least the senses of touch, taste, smell, hearing and sight, it is 

 also certain that these senses of theirs differ remarkably in range from 

 our own, as we have shown. We can form no accurate conception of 

 these ordinary senses in insects, to say nothing of others that insects have, 

 some of which are probably peculiar to insects. Thus they have many 

 curious integumentary organs which from their structure and nerve con- 

 nections are probably sensory end-organs, though their functions are 

 either doubtful or unknown. Such an organ is the sensillum placodeum 

 (p. 76), the use of which is very doubtful, though the organ is possibly 

 affected by air pressure. Insects are extremely sensitive to variations of 

 wind, temperature, moisture and atmospheric pressure, and very likely 

 have special end-organs for the perception of these variations; indeed, 

 the sensilla trichodea are probably affected by the wind, as we have said. 

 The halteres of Diptera, representing the hind wings, contain sensory 

 organs of some sort. They have been variously regarded as olfactory 

 (Lee), auditory (Graber), and as organs of equilibration. When one or 

 both halteres are removed, the fly can no longer maintain its equilibrium 

 in the air, and Weinland holds that the direction of flight is affected by 

 the movements of these " balancers." 



6. DIGESTIVE SYSTEM 



The alimentary tract in its simplest form is to be seen in Thysanura, 

 Collembola and most larvae, in which (Fig. 144) it is a simple tube ex- 

 tending along the axis of the body and consisting of three regions, namely, 



H 



FIG. 144. Alimentary tract of a collembolan, Orchcsella. F, fore gut; H, hind gut; M, 

 mid gut; c, cardiac valve; cm, circular muscle; Im, longitudinal muscle; p, pharynx; py, 

 pyloric valve. 



ore, mid and hind gut. These regional distinctions are fundamental, as 

 the embryology shows, for the middle region is entodermal in origin and 

 the two others are ectodermal, as appears beyond. 



There are many departures from this primitive condition, and the 



