STRUCTURE. 
553 
employ to obtain oxygen are extremely beautiful, but so various that 
we cannot attempt any general account of them ; the student will find 
a great deal of interesting information in MialPs “Aquatic Insects,” 
and further knowledge can be obtained from the nearest tank. 
The typical respiratory system of the Dipterous larva consists of 
two big tracheal tubes running from tail to thorax, connected with each 
other by cross-tubes and having side branches to the lateral spiracles. 
There is a general tendency for these lateral spiracles to degenerate, so 
that breathing may be mainly or entirely carried on by the anterior 
(prothoracic) and posterior spiracles only ; larvae with this arrangement 
are called 4 4 amphipneustic ; ” in many larvae, especially in aquatic 
species, the posterior spiracles alone remain functional, and then the 
larvae are called 44 metapneustic.” In these aquatic forms respiration is 
often carried on in part by gill-processes, generally situated at the tail 
end of the body (see p. 131). 
Some parasitic larvae ( Oestridce ) are able to live for long periods in 
the body of their victim without any air. 
Venation .—The nomenclature of the wing 4 ‘veins ” or “nervures” 
has for long been a source of confusion to the student, since there is no 
one authoritative system in vogue. The most satisfactory is that of 
Comstock and Needham, a modified form of which we have therefore 
adopted. Comstock’s system is based on an extensive comparative 
study of the development of the veins in the wings of a large number of 
insects, and is applicable to other orders besides Diptera. To explain 
it, we will take as an example one of the common large horse-flies ( Taba- 
nus), shown on PL LXII (not the well-known flat brown cattle and 
dog-flies). Forming the front edge of the wing is a strong vein called 
the Costa, which in this case is continued right round the edge of the 
wing. Just behind it is the subcosta, which joins it about half-way along 
the front edge of the wing. Just near the base of the wing the costa 
and subcosta are connected by a short cross-vein, the humeral cross-vein. 
Behind the subcosta is a third vein, which is seen to be single near the 
base of the wing, but branches towards the tip into four separate veins. 
This vein is called the Radius, with its branches, first, second, third 
and fourth (counted from the front). Behind the radius, and connected 
with it by a little cross-vein in the middle of the wing (the anterior cross¬ 
vein), is the Media, which divides near the middle of the wing into an 
