:86 OUR HOUSEHOLD INSECTS 



the whole head is readily rotated round it through an 

 angle of more than ninety degrees either way ; hence 

 often happens in the dead and dried specimen a catas- 

 trophe most unwelcome to the possessor of a collection ; 

 a jar or jerk, or an accidental touch, instantly severs 

 the head from the body and ruins the specimen. This 

 unenviable notoriety flies share with dragon flies. Under 

 the microscope the head is a really handsome object, 

 especially when viewed with reflected light. The multi- 

 tude of detail, the quaintness of form of many of the 

 parts, and the harmony of coloration throughout, com- 

 bine to make it an object to which only a long and 

 attentive examination can do justice. 



The sides are almost entirely occupied with the 

 enormous and strongly convex masses of the compound 

 eyes (Fig. 59), of a colour varying from bright red to 

 dark brown. This appearance is caused by the pigment 

 beneath showing through the transparent cornea; be- 

 hind, they are delicately bordered with silvery tomentum. 

 The facets, very numerous, and for the most part hexa- 

 gonal in outline (Plate I.), cannot be made out at all with- 

 out the help of a lens, and then only when a good light is 

 playing on them. Their form is best seen by stripping 

 off the transparent cornea, and examining it with trans- 

 mitted light. It depends on the sex what proportion 

 the area of these eyes bears to the whole surface of the 

 head. In the males the eyes spread farther towards the 

 centre of the face than in their partners, and in some 

 flies, as, e.g., the smaller house fly (H. canicularis) , 

 the eyes of the males come completely into contact on 

 the forehead, those of the females remaioing some 

 distance apart. Flies whose eyes meet in this way are 

 said to be " holoptic " (whole-eyed). 



On the summit of the head, or crown as it is called, 



