116 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 larvee, in which (Fig. 144) 
it is a simple tube extending along the axis of the body and 
Fic. 145. 
Alimentary tract of a grasshopper, Melanoplus differentialis. c, colon; cr, crop; 
gc, gc, gastric ceca; i, ileum; m, mid intestine, or stomach; mt, Malpighian, or kid- 
ney, tubes; 0, esophagus; 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 (stomodeum) : mouth, pharynx, cesophagus, 
crop, proventriculus (gizzard), cardiac valve. 
