THE DEVELOPMENT OF INSECTS FROM THE EGG
Although this and the following chapter may not be interesting to all my readers, I think it is only right to add some remarks on the structure and classification of insects, so that any one who wishes to follow up the subject may gather a few general ideas which may induce them to take up some technical and scientific work in which they will get fuller and more exact data on the difficulties which are involved in such simple questions as "What is an insect?" "How are the different orders of insects distinguished from each other?" "What is a species?" etc.
To realize the characters of an insect in its perfect or "imago" state, we may for the moment forget what often seems to be its most important features, and which are frequently its most extensive parts, viz. its limbs or
appendages; by limbs are meant its wings, legs, horns or antennæ, jaws or mandibles, etc.: strip these all off, and we have a limbless trunk, which many would not recognize as belonging to an insect at all; still this limbless trunk possesses characters which assert its insect nature, as it may be known from other limbless trunks by being divided into three parts by two great transverse divisions; in most insects these are extremely well marked, and in all they have a very real existence. The parts thus divided off are known by the names of head, thorax, and abdomen. Anybody knows how easy it is to break off the head or body of a dried insect. Now the head or body breaks off at one of these divisions, and it is this partitioning of the body into three sections which makes one of the strongest characters in the definition of an insect. The three parts, thus divided off, each possesses special functions in the life of the creature. In the head are contained the principal organs of sense and brain; in the thorax, the organs of locomotion; and in the body those of digestion, reproduction, etc.
This division into three parts does not however
always hold good in the early stages of the insect's life, and we must remember that the creature commences life on leaving the egg, and not merely on its emergence from the chrysalis, so that we have to reckon with caterpillars, grubs and all sorts of curious immature forms in our conceptions of an insect.
These early stages do not as a rule interest the public much, but it is well to bear in mind that the "perfect insect" stage is reached by some insects along apparently a very different road from that travelled by others. Some leave the egg as caterpillars or grubs, and after various changes of skin become apparently lifeless chrysalids, from which they emerge as perfect insects. Others leave the egg as diminutive likenesses of their parents, and run or hop about much as they do, attaining the perfect insect stage simply by a series of changes of skin, without any definite quiescent or chrysalis condition.
The observation, therefore, which one often hears that insects never grow, has to be taken with caution; all insects grow in their early stages, but it is an obvious truth that insects do not
grow after they attain the imago or "perfect insect" condition. A small fly will never become a large fly, nor a small beetle a large beetle. This is only because we do not recognize their caterpillars or grubs as flies and beetles; but a grasshopper we know grows, because its early stages are of the same general form as the perfect insect, and we see the little ones hopping about in some places, and if we visit the same place later on we notice that they have grown, but as soon as they cast their last skin and obtain the free use of their wings, growth ceases, as it does in a fly or a beetle or in any other insect.
It must not be supposed that the limbs of insects are of no value in their identification. We only removed them in order to emphasize the great importance of the character derived from the regional constrictions of the body, which is considered to be certainly one of the most, if not the most, important of any. Besides this character every perfect insect should have six legs, four wings, and various appendages on the head, such as antennæ, mandibles, maxillæ, labium, etc.; some of these may be so modified as hardly to
be recognizable, but they are hardly ever absent altogether; for instance, the two fore wings of a beetle are modified into what are called wing cases, and fold over its back, protecting the two hind wings, which are more or less membranous, as are those of a bee. They have not the functions of locomotive organs, and are used in flight as poisers. Again in the case of a fly, the hind wings seem to be absent, but they are considered to be represented by two little projecting organs which look like large headed pins or nails, but which are quite useless for locomotive purposes.
The organs of the mouth are especially liable to modification, and on these the older authors used to frame their classification. Insects were divided by them, primarily, into two great divisions, viz. those which had a biting and those which had a sucking mouth; treated in this way, the following orders fall into the division with biting mouths:—
Coleoptera, or beetles; Hymenoptera, or bees, wasps, ants, etc.; Orthoptera and Neuroptera, which include the grasshoppers, earwigs, cockroaches, dragonflies, May flies, etc.
And into the division with sucking mouths:—
Lepidoptera, or butterflies and moths; Diptera or flies, gnats, etc.; Hemiptera, or bugs, including the plant-lice, etc.
These divisions, however, have not been found to be very satisfactory, although very simple when dealing only with the perfect insect stage. In the first place, being framed on this stage only, they are not always applicable to the earlier phases of the insect's life—for instance, although a butterfly or moth has a sucking proboscis, their caterpillars have strong biting jaws, as any gardener well knows. Also bees, wasps, etc., rather upset the arrangement, as they have not only a sucking mouth but also strong biting jaws.
This system of classification has therefore been discarded by most entomologists in favour of that based on the difference between those insects which pass through the distinctive stages of caterpillar and chrysalis on the one hand, and those which emerge from the egg as diminutive likenesses of their parents on the other. In this arrangement, the Coleoptera, Hymenoptera, Lepidoptera, Diptera and Neuroptera, fall into the
first division, or Heteromorphæ as they are called; and the Hemiptera and Orthoptera into the second or Homomorphæ. The dragonflies are the only slightly discordant elements in this arrangement, as, although their larvæ have six legs and walk about under the water and never assume an actual chrysalis condition, still they can hardly be said to resemble their gorgeously coloured parents which fly about so majestically over our ponds, etc.; still this is only one of the many cases which show that nature cannot be held down by any of the arbitrary rules we make for her classification.
The Hymenoptera are therefore characterized and distinguished from other insects by having both a biting and sucking mouth, four clear wings, and by passing through the distinctive liveries of caterpillar or grub, and chrysalis or nymph. It is with this order only with which we have been dealing. To distinguish the aculeate section from the many other forms of the Hymenoptera is too complex a task to undertake here, but the presence of a narrow waist between the thorax and the body, the number of joints in the antennæ never exceeding thirteen in
the male, twelve in the female, and the presence of a sting capable of ejecting poison in this latter sex, are the most prominent features by which the aculeates may be recognized.