THE THORAX AND ITS APPENDAGES. 
13 
together. These pieces are more distinct in some insects than in others, 
but they can always be distinguished by impressed lines upon the sur- 
face called sutures. The three pieces of the thorax are distinguished as 
the tore-thorax, the middle-thorax, and the hind thorax; or, in scientific 
language, the pro-thorax, the meso-th -rax, and the meta-thorax. In the 
Coleoptera the pro-thorax is very large, and forms the large upper part, 
or shield, to which we usually give the general name of thorax. In this 
order of insects, the meta-thorax is invisible above, and the only part 
of the meso-thorax seen from above is the triangular piece between the 
bases of the elytra, called the scutellum. 
In many insects ( Hymenoptera and Eepidoptera) the pro-thorax is 
much reduced in size, and forms only a narrow rim, which is usually 
called the collar. 
The under side of the thorax is called the sternum or breast plate. 
Each of the three divisions of the thorax has its sternum, designated 
respectively as the pro-, meso- and meta-sternum. In many insects, 
and especially the Coleoptera, each section of the sternum is divided 
by sutures into a middle piece or sternum proper, and a side piece, called 
the cpisternum. These parts will be described more particularly in 
treating of the Coleoptera. 
The appendages of the thorax are the organs of potion, namely, the 
wings and the legs. 
The Wings . — The great majority of insects have four wings. The 
anterior pair are attached to the upper part of the meso-thorax, and the 
posterior pair to the meta-thorax. 
The wings are thin, membraneous, transparent organs, in some cases 
folded when at rest, and supported by ribs or veins running across them. 
These veins are found to correspond in their number and complexity to 
the rank of the insect in the scale, and from the ease with which they 
can be seen, they furnish admirable characters for the purposes of class- 
ification. In some insects, such as the grass-hoppers, the fore- wings are 
thicker and less transparent than the hinder pair, and have nearly the 
consistency of parchment ; and in one large order of insects, the Cole- 
optera or beetles, the fore-wings become converted into the hard opaque 
pieces, known as the elytra or wing-cases. The elytra take no part in 
flight, but serve only to cover and protect the hinder or true wings, 
which are folded under them when at rest. 
In one large order, the insects have but two wings, and are named 
from this character Dipteru , or two-winged insects. In these insects the 
place of the hind-wings is supplied by a pair of little knobbed append- 
ages called halteres or poisers. 
There are a few exceptional cases of two-winged insects in some of 
the other orders — for example, some of the smaller Day-flies (Ephemera;) 
in the order of Neuroptera, and the males of the Bark-lice (Coccidw) in 
the order of llomoptera. 
