ANATOMY AND PHYSIOLOGY 



1 01 



this does not prove that the colors are of no assistance to the insect, 

 though it does show that they are not the sole attraction the odor also 

 being an important guide. The honey bee is able to distinguish color 

 patterns, according to the experiments of C. H. Turner. 



Problematical Sense Organs. As all our ideas in regard to the 

 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 connections are inferred to be sensory end-organs, though their 

 functions are either doubtful or unknown. Such an organ is the sensil- 

 lum placodeum (p. 84), the use of which is very doubtful, though the 

 organ is possibly affected by air pressure. Insects are extremely sensi- 

 tive 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. 146) it is a simple tube ex- 



FIG. 146. Alimentary tract of a collembolan, Orchesella. F, fore gut; H, hind gut; M , 

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

 pyloric valve. 



