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. 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 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).
Fig. I.—Body of an insect (Hymenoptera), showing the principal divisions: A, head; B, thorax; C, abdomen; a, antenna; c, compound eyes; m, mandible; s, simple eyes; b, prothorax; d, mesothorax; k, metathorax; 1W, fore-wing; 2W, hind-wing; n, coxa; o, trochanter; p, femur; r, tibia; t, tarsus; 1 to 9 segments of the abdomen.
Fig. II.—Oral and digestive system of Deinacrida megacephala (this insect is drawn on Plate XVIII., fig. 2): 1, mandibles; 2, maxillæ; 3, labrum; 4, labium; 5, maxillary palpi; 6, labial palpi; 8, œsophagus; 9, crop; 10, gizzard; 11, pancreas; 12, stomach; 13, biliary vessels; 14, ilium; 15, colon; 16, anus.
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 œsophagus (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.
The eggs of these animals exhibit a great diversity in shape among the different species. They are deposited by the parent with unerring instinct on substances suitable for the food of the larvæ, which, in the majority of cases, is quite different from that on which she herself subsists.
The larva state immediately succeeds the egg, and is spent almost exclusively in feeding, the insect growing at a great rate, and being frequently compelled to change its skin.
The pupa is usually completely quiescent, the insect being at this time quite incapable of any motion, except, perhaps, a slight twirling of its abdomen. Exceptions to this rule occur, however, in two of the orders, in which the pupa state does not differ materially from that preceding it.
In the imago, or perfect state, the insect appears under its final form, with every organ completely developed.
We will now consider the seven great divisions, or Orders, into which insects are divided, the complete knowledge of which is one of the most important elements in the entomologist's preliminary education. I trust that by a careful perusal of the following definitions, aided by references to the Plates, which illustrate numerous members of each order in their several states, the reader will be enabled to master the subject without much difficulty.
Wings four; the anterior pair (termed elytra) horny and opaque, the posterior membranous, and employed in flight; mouth masticatory. The larva a grub with or without legs, but a distinct head always present. The pupa inactive, taking no food, the limbs of the future insect enclosed in distinct cases, and applied closely to the body. This is the largest of the Orders, and consists of all those insects popularly known as Beetles. (Plates I. and II.)
Wings four, membranous, the posterior pair being the smaller, and connected with the anterior during flight by a row of minute hooklets; mouth masticatory, the maxillæ and labium being elongated, in many of the families, into a long sucking instrument or "tongue." Metamorphosis as in the Coleoptera. A large Order, containing the numerous tribes of Sawflies, Bees, Wasps, Ants, and Ichneumon-flies. (Plate III.)
Wings two; the posterior pair represented by two minute clubbed appendages termed poisers; mouth a suctorial tube formed by an elongation of the labium, enclosing within it a variable number of setæ answering to the mandibles, &c., of biting insects. The larva without legs, a distinct head being often absent. The pupa inactive, the limbs of the imago firmly attached to the body, but plainly visible. Among the majority of species included in this Order the larval skin is not cast away, but envelopes the insect in a hard shell; the true pupa is consequently only visible on the removal of this covering, when it is found to closely resemble those in which no such arrangement occurs. The Order comprises the numerous Gnats and two-winged Flies. (Plates IV., V., VI., VII.)
Wings four, generally covered with scales; the anterior pair slightly superior in size; mouth suctorial, the maxillæ forming a spiral tongue, which is coiled between the large labial palpi when not in use; other oral organs rudimentary. In many instances the whole mouth and alimentary canal are more or less obliterated, a considerable number of the species taking no food in their final state. The larvæ always possess a distinct head and six thoracic legs, and in addition a variable number of prolegs are often present on the abdominal segments. Pupa inactive, the limbs of the future insect being usually indicated by lines in the integment. This Order contains all the varied tribes of Butterflies and Moths. (Plates VIII., IX., X., XI., XII., XIII.)
Wings four, of equal size, membranous, and traversed with numerous branching ribs; the mouth masticatory, and in many instances but slightly developed. Larva with a distinct head and three strong thoracic legs; chiefly carnivorous. Pupa inactive; the limbs very perceptible and loosely applied to the body, but incapable of distinct motion. A small Order, comprising the Stoneflies, Lace-wings, Ant-lions, &c. (Plate XIV.)
Wings four, of nearly equal size; the anterior pair often more or less leathery, but with distinct veins. The larva and pupa closely resembling the imago; the latter with rudimentary wings. In the instances where these organs are wanting in the mature insect, the metamorphosis merely consists of a series of moultings, and it is consequently a matter of some difficulty to determine when the insect is full-grown. This Order is of small extent; it includes the Earwigs, Cockroaches, Grasshoppers, Crickets, Termites, Dragonflies, Mayflies and Perlidæ; the last four being transferred from the Neuroptera of most authors. The minute species of Mallophaga and Thysanura will also come under this heading. (Plates XV., XVI., XVII., XVIII., XIX.)
Wings four, in some cases wholly membranous, but in a large proportion of the families the basal portions of the anterior pair are horny, and form protective cases for the other pair when not in use; mouth suctorial, consisting of an elongate rostrum, enclosing four fine setæ. The larva and pupa resemble the imago, the latter being active, with rudimentary wings. In a few instances, a slight divergence from the parent form is shown in the preparatory states (Cicadas, &c.). This is a small Order, containing the Cicadas or "Singers," Bugs, Plant Lice, and all the suctorial animal lice. (Plate XX.)
After the Orders, the divisions to be considered are the Groups, Families, Genera and Species.
Groups are large divisions immediately subordinate to the orders, and consist of a number of kindred families. They are of great assistance to the student in dealing with the very large Orders, such, for instance, as the Coleoptera.
Families, again, consist of a number of allied genera, and Genera, in the same way, of allied species.
With regard to the Families, I have in the main followed those of Professor Westwood in his 'Modern Classification of Insects,' as most recent writers appear very much divided in opinion as to the correct limits of these divisions. Much diversity also prevails with respect to the proper definitions of Genera and even Species, but I have deemed it best to follow the authority of the latest catalogues in this matter, as any changes in nomenclature are always liable to produce confusion.