INSECTA. 161 
discharge the functions of salivary glands. Immediately behind 
the posterior aperture of the stomach are a variable number of 
czecal convoluted tubes (¢), which are known as the “ Malpighian 
vessels,” after their discoverer Malpighi, and which are generally 
looked upon as representing the liver, but which may correspond 
with the kidneys of higher animals. Close to the cloaca may 
be other tubes, which are believed, from their position, to exer- 
cise the functions of kidneys (Z). 
The circulation in insects is mainly carried on by a long con- 
tractile tube placed along the back, and termed the “ dorsal 
vessel.” The blood, collected from the various tissues and 
organs of the body, enters the dorsal vessel from behind, and 
is driven forwards to the anterior extremity of the body. Re- 
spiration is effected by means of air-tubes or trachezs, which 
commence at the surface by so many apertures or spiracles, and 
branch repeatedly as they proceed inwards through the tissues. 
They have the same structure as in the Arachnida, consisting 
of membranous tubes strengthened by means of a spirally-coiled 
filament of chitine. 
The nervous system in insects, though sometimes somewhat 
modified, has essentially the regular annulose form of a ventral 
chain of ganglia, traversed in front by the gullet. The organs 
of sense are the eyes and antennz. The eyes are usually “com- 
pound,” and are composed of numerous six-sided lenses, united 
together, and each supplied by a separate nervous filament. 
As many as eight thousand of these lenses have been counted 
in one of the eyes of the common cockchafer, and this number 
is sometimes greatly exceeded. Besides these compound eyes 
there are sometimes “simple” eyes, identical in structure with 
the single lenses of the compound eyes ; and in rare cases these 
are the only organs of vision. The feelers or antennz, with 
which all ‘insects are furnished, are jointed filaments attached 
close to the eyes, and assuming very different shapes in different 
insects. They appear to be certainly organs of touch, but they 
very probably minister to other senses as well, and there is 
some reason to suppose that they are connected with the sense 
of hearing in particular. 
_ The sexes in insects are distinct, and most of them are ovipa- 
rous. Generally speaking, the young insect is extremely dif- 
ferent in external character from the adult, and it requires, 
before reaching maturity, to pass through a series of changes 
L 
