The Structure and Special Physiology of Insects 11 
ence in the wings within a single order; most beetles, for example, have 
four wings, but some have two and some none. There are indeed wingless 
species in almost every insect order. But a typical beetle has quite dis¬ 
tinctive and commonly recognized wing characters; that is, it has two pairs 
of wings, the fore pair being greatly thickened, and developed to serve as 
sheaths for the larger, membranous under-pair, which are the true flight 
wings. Similarly, practically all moths and butterflies have two pairs of 
Fig. 17. Fig. 18. 
Fig. 17. —Wing of cabbage-butterfly, Pier is rapes, in early sac-like stage, tr., trachea; 
tl., tracheoles; l.v., lines of future veins. (After Mercer; greatly magnified.) 
Fig. 18. —Diagram of wings of monarch butterfly, Anosia plexippus, showing venation. 
c costal vein; s.c., subcostal vein; r., radial vein; cu., cubital vein; a ., anal veins. 
In addition, most insects have a vein lying between the subcostal and radial veins, 
called the median vein. (Natural size.) 
membranous wings completely covered above and below by small scales, 
which give them their distinctive color and pattern. 
The exoskeleton, or cuticle, of the insect body is sometimes nearly 
smooth and naked, but usually it is sculptured by grooves and ridges, punc¬ 
tures or projections, and clothed with hairs or those modified flattened hairs 
known as scales (especially characteristic of butterflies and moths). This 
clothing of hairs or scales, or the skin itself, is variously colored and pat¬ 
terned, often with the obvious use of producing protective resemblance or 
mimicry, but often without apparent significance. (For an account of the colors 
and patterns of insects and their uses see Chapter XVII.) The hairs may serve 
for protection, or may be tactile organs, or even organs of hearing (see p. 26). 
The projecting processes may be spines or thorns or curious and inexplicable 
