184 M i c r o s c o p i c a l E s s a y s .
Serrata, having the external margin edged with teeth or
notches.
Spinosa, when the exterior furface is covered with fmall (harp
points.
ScabrA, when it is very rough.
S t r ia t a , marked with (lender longitudinal furrows.
Porcati, having (harp longitudinal ridges.
Sulcati, with deep furrows.
H e m e l y t r a , when the cafes are neither fo hard as the elytra,
nor fo delicate as the tranfparent wing.
Under the wings of mod infefts, which have only two, there
is a fmall head placed on a (talk, frequently under a little
arched fcale ; thefe are called halteres, poifers ; they appear to
be rudiments of the hinder wings: it has been fuppofed that they
ferve to keep thé body in eqnilibrio when the infect is flying.
The bodies of infefts are covered with a hard (kin, which an-
fwers the purpofe of an internal (keleton, and is one of the dif-
tinguifhing charafters of. an infeft. All quadrupedes, birds, and
fifties, have an internal (keleton o f bones to which the mufcles
are fixed; but the whole interior body o f an infeft is compofed
o f foft flefh, and the mufcles are affixed to the exterior (keleton
or
M i c r o s c o p i c a l E s s a y s . 185
or (kin, which in thefe anfwer the purpofe of bones. The (kin
ferves as a continent to the whole infeft, covering the body, con-
nefting the parts, and maintaining them in their proper places.
This external covering is very ftrong in thofe infefts, which, by
the nature o f their life, are expofed to ftrong friftion, or violent
cotnpreffion ; but is more tender and delicate in thofe which are
not fo expofed. The (kin of infefts, like that o f larger animals,
is porous; the pores in fome fpecies are very large. Many
in lefts often change or caft off their (kin; this exuvia forms an
excellent objefi for the microfcope.
Another diftinguiftiing criterion o f infefts is the colour o f their
blood, which -is never red; this, at firft fight, feems liable to
fome objeftions, on account of the drop o f red liquor which is
often procured from fmall infefts when fqueezed or prefled to
pieces. It does not appear, however, that this is the blood o f
the little animal; when it exifted as a worm there was no fuch
appearance, and when they exift as a fly it is only found in the
eye, and not in the body, which would be the cafe if it circulated
in the veins of the infefts. It is probable there is a circulation o f
fome fluid analagous to the blood in mod infefts: with the
afliftance of the microfcope this circulation may-be perceived in
many; but the circulating liquor is not red.
T o thefe difcriminating charafteriftics we may alfo add the following
particulars: 1. That the body o f infefts is divided by
incifiirae, or tranfverfal divifions, from whence they take their
name.
Z 2. That