66 
HOW TO COLLECT 
head in the male, but are move or less widely separated in the 
female. With the exception, however, of a few small families, 
this does not apply to the Orthorrhapha Nematocera [Gall-midges 
( Cecidomyidce ), Fungus-midges ( Mycetophilidce ), Chironomidw, 
Culicidce, and l'ipulidce], the Diptera that are gnatlike in shape 
and possess long antennae ; neither is the character found in the 
Robber-flies ( Asilidce ), the Dolichopodidce (shining green, narrow¬ 
bodied, long-legged flies, usually found in the vicinity of water), nor 
in the vast army of Diptera (chiefly of small or moderate size) 
included in the families which it is customary to designate compre¬ 
hensively by the term Acalypterate Muscidce. In these cases, where 
no sexual distinction is furnished by the eyes, it is usually possible 
to determine the sex by an examination of the terminal segments 
of the abdomen : in the male the abdomen ends more bluntly, while 
the clasping organs are often very conspicuous; in the female the 
tip of the abdomen is generally more or less tapering, showing 
indications of the presence of an ovipositor, which is often extruded. 
In the Mosquitoes (Culicidce *) the sexes can be readily distinguished 
by the antennse, which in the male are plumose, forming tufts in 
front of the head, while in the female they are nearly bare; in the 
Midges also ( Chironomidce ) the antennae in the male form plumes, 
while in the female they are quite short and inconspicuous. In 
certain families [Dolichopodidm, Empidce) secondary sexual characters 
are often furnished by the legs; where this is the case, it usually 
takes the shape of abnormal expansions of some of the tarsal joints 
in the male. 
As a rule among Diptera the sexes of the same species are alike in 
coloration, or at any rate resemble one another pretty closely, 
striking sexual differences in this respect (such as are commonly 
seen among butterflies) being exceedingly rare. 
When and Where to look for Diptera. 
With the exception of Mosquitoes, Midges, and similar forms, 
Diptera are most active when the sun is shining. In hot countries 
* For collecting this family, see special instructions (“ llow to Collect 
Mosquitoes”), pp. 81-91, 
