THE HABITS AND STRUCTURE OF MOTHS. xxiii 
than the breadth of the wings. The fringes have generally 
regular markings, and are sometimes intersected by one or 
more dark lines, parallel to the hind margin. The hind 
margin may be straight, concave, sinuated, dentated, &c., 
terms which sufficiently explain themselves. 
The wings are composed of a stout membrane, spread over 
a system of hollow ribs or nervures, much in the same way 
as an umbrella is stretched. The neuration is of great im¬ 
portance, as it not only enables us accurately to define any 
portion of the wing, but the arrangement of the nervures 
forms a valuable character in tracing the natural position 
in which an insect should be placed. 
Two principal nervures arise from the base of each wing, 
which are usually connected, at or beyond the middle of the 
wing, by a short transverse nervule, 1 which is often angu- 
lated or curved towards the base, and thus is enclosed an 
area which is called the discoidal cell. These two principal 
nervures are called the subcostal and the median nervures. 
From these, and from the transverse nervule, arise a number 
of other nervules, which end in the hind margin and costa. 
These are numbered from behind forwards 2, 3, 4, &c., 
whether they arise separately from the median nervures and 
the transverse nervule, or whether two or more spring from 
a single stem. 
In addition to these there are from one to three nervures 
towards the inner margin of the wings (only one usually on 
the fore wings), which arise from the base and run to the 
hind or inner margin. These are the submedian or internal 
nervures, and are numbered la, lb, 1 c, from behind for¬ 
wards. There is another nervure running from the base to 
the costa, which is called the costal nervure, and which is 
sometimes absent on the hind wings. On the hind wings of 
many moths this nervure is connected with the subcostal 
nervure for a short distance near the base, or is merged with 
it near the base, so that it seems to form part of the sub¬ 
costal nervure. The frenulum, mentioned above, is really a 
modified nervure disconnected with the membrane. In most 
Lepidoptera in which the frenulum is wanting, there are, 
instead, one or more short curved nervures running into the 
costa of the hind wing. Nervure 5 of the hind wings is 
often wanting, or more slender than the others; sometimes, 
also, nervure 5 is wanting on the fore wings. In this case 
its position is easily recognised by the greater distance be¬ 
tween nervures 4 and 6, and this, as well as any other ner- 
1 Nervures which do not arise from the base of the wing are generally 
called nervules. 
