INTRODUCTION, 
549 
produce their young as larvae or 
pupae. 
(6) Siphonaptera, Fleas. Parasi¬ 
tic wingless jumping insects with 
narrow flat-sided bodies and lea¬ 
thery skin. 
These two groups are so much modified in accordance with their 
parasitic mode of life that they are easily distinguished from other 
Diptera, and we shall at present consider only the first four. The classi¬ 
fication of these given above depends on the structure of antennae, palpi, 
and head, but there is another character, the venation, which is of con¬ 
siderable importance (See p. 553). 
The life-histories, while very various in detail, exhibit a general 
broad similarity. The larvae in the Nemocera are mostly aquatic or 
inhabit wet places, the former usually developing into floating pupae. 
Very few of the larvae are predaceous, most of them being vegetarians 
or scavengers, and the imagos are in many cases blood-suckers (Mos¬ 
quitos, Midges, Sandflies, and Simulium). One family, the Cecidomyiidce, 
includes many gall-makers. The larvae of the Brachycera are only 
occasionally semi-aquatic or aquatic {e.g., Tabanidce, the horse-flies, 
and Stratiomyidce) and are mostly either predaceous or parasitic on 
other insects ; a few are scavengers. The adult Tabanids suck the blood 
of animals ; several other families suck the juices of insects, while those 
flies which develop from parasitic larvae generally frequent flowers and 
suck nectar. 
In both the Nemocera and Brachycera most of the larvae have a 
distinct and well-formed head which is clearly separate from the tho¬ 
racic segments. 
The larvae of Aschiza and Schizophora are of a different type, for in 
these the head is very small and either habitually retracted or poorly 
developed, without any neck or distinctly marked division from the 
thorax, while the jaws are generally represented by two small hooks 
which work up and down, and not horizontally as in many Nemocera- 
With the exception of some Syrphidce we do not know of any 
predaceous larvae in these groups ; a few are aquatic, a considerable 
number attack living plants, and a still larger proportion are scavengers 
or are parasitic on insects or mammals. 
