An elementary manual of New Zealand entomology — Key Ideas to Explore

GENERAL OBSERVATIONS 1

EXPLANATION OF PLATES 129

NEW ZEALAND ENTOMOLOGY.

General Observations.

In the present chapter I propose to give a brief sketch of the general principles of Entomology, including a rudimentary glance at the anatomy and classification of insects; after which I think the reader will be in a better position to study the habits and life-histories of the individual species which follow.

The first requisite is a definition of what constitutes an INSECT.

_An Insect is an articulate animal having the body divided into three distinct divisions_, viz., _the_ HEAD (Fig. I. A), _the_ THORAX (B), _and the_ ABDOMEN (C). _It is furnished with three pairs of legs, and generally has two pairs of wings, and to acquire this structure the creature passes through several changes, termed its metamorphoses._ {2}The head exhibits no distinct divisions, but bears the following appendages: the eyes, antennæ, and organs of the mouth, or trophi.

The eyes are of two kinds, compound and simple. The former (Fig. I. c c) are situated on the sides of the head above the mouth, and consist of two large hemispheres, composed of a great number of hexagonal divisions, each of which is a complete eye in itself. The latter (s s) are usually three in number, and are situated on the top of the head between the compound ones. They are, however, frequently wanting.

The antennæ (a) are two jointed organs, one of which is placed on each side of the head, between the eyes; their functions are at present extremely doubtful, but they are invariably found in all insects.

The organs of the mouth consist of the following: the labrum (Fig. II. 3), or upper lip, a horny plate, closing the mouth from above; the mandibles (1 1), or upper jaws, two strong bent hooks, articulated to the head on each side of the mouth, and opposed to one another like scissor blades; the maxillæ (2 2), or under jaws, resembling the mandibles, but more delicately constructed, and furnished with a pair of jointed appendages termed maxillary palpi (5 5); and the labium (4), or lower lip, consisting of a horny plate somewhat resembling the labrum, but provided with two jointed appendages termed the labial palpi (6 6). All these organs are subject to great modification in suctorial insects, which I shall notice further on, when dealing with the differences between the various orders.

The thorax consists of three primary divisions, viz., the prothorax (Fig. I. b), mesothorax (d), and metathorax (k). The upper surfaces of these are termed the pronotum, mesonotum, and metanotum respectively, and the under the prosternum, mesosternum, and metasternum; other divisions exist in some insects, but they are not of a sufficiently {3}general character to be noticed here. The six legs are attached to the under surface of the thorax, a single pair to each division; they are composed of the following joints: coxa (Fig. I. n), trochanter (o), femur (p), tibia (r), and tarsus (s).

{4}The wings are attached to the meso- and metanotum; they consist of two membranes traversed by numerous horny ribs (Fig. I. 1W and 2W).

The abdomen is made up of nine segments (C 1 to 9), some of which are not infrequently wanting. It contains the organs of nutrition, circulation, and generation.

The digestive system, the structure of which is apparent from Fig. II., consists of the following divisions: the throat, or oesophagus (8); the crop (9); the gizzard, or proventriculus (10); the pancreas (11 11); the stomach, or ventriculus (12); the biliary vessels (13 13 13); the ilium, or little gut (14 14); and the colon (15); ending in the anus (16). In the suctorial tribes, the crop is modified into a very peculiar organ, termed the sucking stomach, which presents itself as a small bag, attached to the throat by a thin tube. This bag exhausts the air from the throat, when the insect is sucking, thus producing a vacuum therein, and causing a rapid ascent of fluid into the stomach.

The heart of insects consists of an elongated tube lying along the back, and termed the dorsal vessel. It is composed of a variable number of chambers, the blood being driven forward towards the head by its contractions. These motions may be easily seen in transparent species.

The breathing organs are distributed throughout the body in the form of numerous minute air-tubes, which are supplied with air from a variable number of apertures, situated on the sides of the insect, and termed spiracles.

The nervous system consists of a chain of ganglia, running down the ventral surface of the insect, and analogous to the spinal cord of higher animals. The number of ganglia varies greatly among the different tribes.

The metamorphosis of insects, which I have previously mentioned as one of their most essential attributes, consists of four distinct stages, _viz._, the Egg, Larva, Pupa, and Imago.