34 



Notes on the Diptera. 



By H. W. Andrews, F.E.S. Read October 28///, 1909. 



I PROPOSE to-night to make some general remarks about the 

 Diptera which may perhaps attract a recruit or two to the slowly 

 growing numbers of British Dipterists, by pointing out some of the 

 various ways whereby the study of this order may be approached. 



The Diptera is one of the " neglected " orders. The word " flies " 

 brings to the mind of the non-entomological public the annoyance of 

 house-fly and bluebottle, with, perhaps, thoughts of daddy-longlegs 

 and gnats; and, judging from my own- experience, it does not repre- 

 sent very much more to the average lepidopterist. As a matter of 

 fact it yields to none of the other orders of insects in importance ; it 

 contains some of the most highly specialized forms, judging from the 

 completeness of their metamorphoses, that exist among insects ; it 

 exhibits extreme variety in structure and habits; and one has only to 

 recall such terms as the Hessian fly, malaria and mosquitoes, 

 sleeping-sickness, and the tse-tse fly to realise how large a part the 

 Diptera play in economic entomology. It is one of the larger orders 

 in point of numbers ; a total of over 40,000 described species was 

 mentioned by Dr. Sharp in "Cambridge Natural History," published 

 in 1899,* and it is probable that this number will have to be greatly 

 extended. In the British list of between 2000 and 3000, new species 

 are recorded every year, and it is probably due to the scarcity of 

 workers that additions are not made more rapidly. 



An examination of the external forms of the Diptera will give some 

 idea of the remarkable variation and modification of structure that 

 obtain within the limits of the order. As might be expected in an 

 order containing such a large number of aerial species, the eyes are 

 a very prominent feature, in some genera taking up the greater part 

 of the head space. There are, as a rule, three small ocelli, but the 

 facetted eyes are of much more importance ; they are often hairy, 

 more so perhaps in the males than in the females, and in some 

 species are banded or otherwise marked with bright colours during 

 life. In the males of the genus Bibio the eyes are distinctly separated 

 into two portions with different-sized facets ; this division of the 

 facets is found in many other species, though not quite so promi- 

 nently, and like the hairiness, is usually confined to the males. A 

 similar arrangement of facets is found in certain dragon-flies, and it 

 has been supposed that the insect obtains a double vision as it were, 



* " Insects," part ii, p. 438. 



