CHAPTER III 

 SECTION 1 



THE ORDER DIPTERA : ORTHORRAPHA NEMATOCERA 



THE Diptera form a well defined and highly specialized group of insects, 

 easily distinguished, in the great majority of cases, by the presence 

 of only one pair of wings; the second pair of other insects being 

 represented by the halteres. The order is a large one, not less than 

 36,000 forms being known at the present time, while new genera and 

 species are constantly being added. The study of the group has re- 

 ceived a special impetus since the economic importance of many of the 

 species has become recognized. 



Before passing on to the classification and the detailed description 

 of the important forms, it is necessary to refer briefly to the life 

 history and early stages, and to explain certain con- 

 nected terms in common use. Metamorphosis in the Met amorpho8i 8 and 



1 Early Stages 



Diptera is always complete, the animal passing through 



the separate stages of egg, larva, pupa, and imago. The eggs vary 

 greatly in size and appearance, and will be described under the species 

 to which they belong. They are always minute, often invisible to 

 the naked eye or nearly so, and are laid in or near the food of the 

 larva, in numbers which correspond as a rule with the number of ova- 

 rioles in the ovaries. The larvae vary with their habitat and accord- 

 ing to the family to which the imagines belong, the former factor 

 exercising the predominant influence over their- structure. Generally 

 they are footless maggots, with an elongated and segmented body, 

 consisting of a larger number of segments than can be distinguished 

 in the adult. They may be aquatic or semi-aquatic in their habits, 

 or they may live in decaying animal or vegetable matter, or in the 

 excreta of animals. Although it is not necessarily the case that the larvae 

 of allied forms resemble one another, as a general rule they do so, 

 and a correspondence between the systematic position of the imagines 

 and the structure of the larvae can be noted. 



