Il6 ENTOMOLOGY 



to variations of wind, temperature, moisture and atmospheric 

 pressure, and very likely have special end-organs for the per- 

 ception 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, con- 

 tain 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 extending along the axis of the body and 



FIG. 145. 



f c 

 cr 



o-.. 



Alimentary tract of a grasshopper, Melanoplus differ entialis. c, colon; cr, crop; 

 gc> & c > gastric caeca; i, ileum; m, mid intestine, or stomach; mt, Malpighian, or kid- 

 ney, tubes; o, oesophagus; p, pharynx; r, rectum; s, salivary gland of left side. 



consisting of three regions, namely, fore, mid and hind gut. 

 These regional distinctions are fundamental, as the embry- 

 ology 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 most specialized insects exhibit the following modifi- 

 cations (Figs. 145, 146) of the three primary regions: 



Fore intestine (stomodceum) : mouth, pharynx, oesophagus, 

 crop, proventriculus (gizzard), cardiac valve. 



