The larvæ of moths and butterflies are popularly known as caterpillars.
They always consist of thirteen segments, segment number one being the
head. The head is furnished with several simple eyes (Plate I., fig. 2,
AA), a pair of very short antennæ (BB), and a very powerful masticatory
mouth. The mouth consists of the following organs: The labrum, or upper lip
(1); a pair of mandibles, or upper jaws, working like scissor-blades (2,2);
two maxillæ, or lower jaws (3,3), each carrying a jointed organ termed the
maxillary palpus; and the labium, or lower lip (4); which bears another
pair of minute jointed appendages--the labial palpi.
Segments 2, 3, and 4, which answer to the thorax of the perfect insect, are
each furnished with a pair of legs. They consist of the six following
joints (fig. 2): (_a_) coxa, (_b_) trochanter, (_c_) femur, (_d_) tibia,
(_e_) tarsus, and (_f_) claw. These legs correspond to those of the perfect
insect. The remaining nine segments of the body constitute the abdomen.
Usually segments 7 to 9 and 13, each have a pair of fleshy pads, which are
termed prolegs and are furnished on their edges with a row of minute
hooklets (see Plate I., fig. 14, proleg highly magnified). It is these
hooklets which enable caterpillars to hold on by means of their prolegs
with such great tenacity. The number of the prolegs varies considerably in
different groups and families.
The _spiracles_, or orifices of the air-tubes, are situated on each side of
the larva just above the legs. They are usually present on segments 2 and 5
to 12, but vary {x}considerably in different groups and families. The larva
is provided with a very complete digestive system, which consists of the
following organs (see Plate I., fig. 9): A, the oesophagus; D, the
ventriculus; F, the clavate intestine; E, the ilium; H, the colon; K, the
biliary vessels; and O, the spinning vessels. These last open at a small
orifice in the labium termed the spinneret (fig. 2, 5). They supply the
silken threads which are employed by most larvæ in constructing their
cocoons, and which also serve in cases of danger as a rapid means of
retreat. Many larvæ, which live on shrubs and trees, suddenly lower
themselves to the ground by means of one of these silken threads, and thus
often escape being devoured by insectivorous animals.
The entire growth of the insect is accomplished during the larval
condition, the increase in size being frequently very rapid. Owing to this
circumstance larvæ are often compelled to shed their skin, and in many
species a very considerable alteration both in the shape and colour takes
place at each moult, or ecdysis as it is sometimes termed.