36 



the arrangement of veins and cells). They are usually unornamented, 

 though often showing handsome iridescent colours, but in some 

 genera, e.g. the Trypetidce, are covered with intricate patterns not 

 formed with scales, but pigmental. A few families, usually parasitic 

 in habits, are wingless. As is well known, the Diptera possess but a 

 single pair of wings, the place of the hind wings of other orders 

 being taken by a pair of small organs known as halteres, which, with 

 the exception of certain male Coccids, are peculiar to flies. These 

 small drumstick-shaped rods are of use as balancers in flight, as it 

 has been experimentally proved in several cases that when cut off 

 the insect appeared quite unable to control its movements. The 

 halteres are often covered by hood-like organs called squamae. The 

 genitalia agree with the rest of the external parts in being grea-tly 

 variable. The Dolichopodidce. again, as in antennae and legs, afford 

 the most varied examples, and it is rather curious that although 

 great modifications are to be observed in the males, the female 

 genitalia are as a rule extremely simple. The difference has been 

 quaintly summed up by some author with the remark that it is like 

 using a most complicated key to open a lock that could be easily 

 forced by a bent hair-pin. The foregoing remarks will show, I think, 

 that there is a large field here for workers in the study of morphology 

 alone ; excellent work has been done by Mr. Wesche in his micro- 

 scopic studies, but there is room for much further investigation. 



The anatomy of Diptera has been thoroughly worked out in the 

 case of a few species, but of a few species only. I know nothing 

 of this branch of study myself, but the principal points appear to 

 be the extremely specialized arrangement of the nerve ganglia that is 

 found in some of the MuscidcB, the presence of large air-sacs at the 

 base of the abdomen, the presence of a sucking stomach as in the 

 Lepidoptera, the constant number of the Malpighian tubes, and 

 various peculiarities in the reproductive organs. The large thoracic 

 spiracles contain "vocal cords" which give rise to the "crying" 

 sound often noticed in some species when captured, and which 

 arises quite independently of wing movements. There are special 

 nervous structures situated in the joints of the antennte which have 

 been experimentally proved to act as olfactory organs, and others at 

 the base of the halteres supposed to be of an auditory character, or, 

 as other consider, organs of orientation. 



The same remarks apply to the life-histories of Diptera as to the 

 internal structure. In some species, especially those whose larvae 

 are aquatic, the life-histories have been studied from the time of 

 Reaumur onwards ; but, on the other hand, little or nothing is known 

 of the life-histories of many other whole genera. This may be 

 partly due to the fact that most Diptera spend their larval existence 

 in concealment, and, moreover, they are often very unattractive, 

 being, as a rule, mere footless maggots without visible heads. The 

 larvce, however, fully carry out the great variability that I have 

 already remarked on as obtaining generally in the order. They may 



