20 DIPTRRA OF MINNESOTA. 



with fluid : rt, retina just below basilar membrane. Upon this "re- 

 tina" the image is probably formed. It must not be supposed, how- 

 ever, that the House Fly sees as many images of an object as it has 

 facets for such cannot be the case ; nor must we believe that the 

 sight of flies or any other insects is comparable to ours. Just how 

 much a fly can see is a mooted question, and introduces arguments 

 altogether too lengthy for this report. Students are referred to 

 Lowne's "Blow Fly," in two volumes, for an exhaustive discussion 

 of this point, as well as other points which touch on the anatomy 

 and physiology of flies. 



In addition to these compound eyes there are, in most flies, 

 three simple eyes, or ocelli, on the top of the head, between the com- 

 pound eyes. There may be but two of these. While without 

 much doubt they have to do with the discernment of light or 

 shadow, their exact function is unknown. We show in Fig. 8 a 

 longitudinal section of a very much enlarged simple eye. The cells 

 at the bottom of the eye are connected with a nerve, n, and are 

 probably sensitive to light. 



Attached to the front of the head are feelers or antennae. Since 

 in Fig. 4 they lie close to the head, and are therefore not easily 

 seen, the reader is referred to Fig. 9, where various forms of an- 

 tennae peculiar to different families are shown in detail. As seen 

 in this drawing, the third joint bears an appendage called the 

 arista if it is bristle-like, or a style if it is more like a hair. The 

 arista shows in the drawing of the House Fly, Fig. 4. The various 

 shapes and peculiarities in structure of the antennae are important 

 factors in classifying flies. In one division, or sub- 

 order, there is a "crescent shaped piece just above the 

 antennae, called the frontal hinnlc. In most of the 

 members of this sub-order which have a frontal 

 lunule is a suture, separating the lunule ^rom that 

 Fig. 6. Corneal „^j.|^ q£ ^j^g head which lies above it. The palpi (the 



facets of a com- I <- ^'^ >• v- / r \ 



frcT'liew^'mtTch T^'^g^^^ ouc is showu in Fig. 4), are appendages of one 



enlarged. ^f |.j^g mouth parts. the maxilla. 



On the under side, ventral side, of the head, about the mouth 



are the mouth parts. A very few species which take no food in 



the adult stage, have but rudimentary mouth parts. The mouth 



parts of all flies, though variously modified, are adapted for suck- 



