Our Common Insects / A Popular Account of the Insects of Our Fields, Forests, Gardens and Houses

by A. S. Packard

For reasons which we will not pause here to discuss, we have always regarded the eruciform type of larva as the highest. That it is the result of degradation from the Leptus or Campodea form, we should be unwilling to admit, though the maggots of flies have perhaps retrograded from such forms as the larvæ of the mosquitoes and crane flies (Tipulids, Fig. 204).

That the cylindrical form of the bee grub and caterpillar is the result of modification through descent is evident in the caterpillar-like form of the immature Caddis fly (Pl. 3, fig. 2). Here the fundamental characters of the larva are those of the Corydalus and Sialis and Panorpa, types of closely allied groups. The features that remind us of caterpillars are superadded, evidently the result of the peculiar tube-inhabiting habits of the young Caddis fly. In like manner the caterpillar-form is probably the result of the leaf-eating life of a primitive Leptiform larva. In like manner the soft-bodied maggot of the weevil is evidently the result of its living habitually in cavities in nuts and fruits. Did the soft, baggy female Stylops live exposed, like its allies in other families, to an out-of-doors life, its skin would inevitably become hard and chitinous. In these and multitudes of other cases the adaptation of the form of the insect to its mode of life is one of cause and effect, and not a bit less wonderful after we know what induced the change of form.

Having endeavored to show that the caterpillar is a later production than the young, wingless cockroach, with which geological facts harmonize, we have next to account for the origin of a metamorphosis in insects. Here it is necessary to disabuse the reader's mind of the prevalent belief that the terms larva, pupa and imago are fixed and absolute. If we examine at a certain season the nest of a humble bee, we shall find the occupants in every stage of growth from the egg to the pupa, and even to the perfectly formed bee ready to break out of its larval cell. So slight are the differences between the different stages that it is difficult to say where the larval stage ends and the pupa begins, so also where the pupal state ends and the imago begins. The following figures (205-208) will show four of the most characteristic stages of growth, but it should be remembered that there are intermediate stages between. Now we have noticed similar stages in the growth of a moth, though a portion of them are concealed beneath the hard, dense chrysalis skin. The external differences between the larval and pupal states are fixed for a large part of the year in most butterflies and moths, though even in this respect there is every possible variation, some moths or butterflies passing through their transformations in a few weeks, others requiring several months, while still others take a year, the majority of the moths living under ground in the pupa state for eight or nine months. The stages of metamorphosis in the Diptera are no more suddenly acquired than in the bee or butterfly. In all these insects the rudiments of the wings, legs, and even of the ovipositor of the adult exist in the young larva. We have found somewhat similar intermediate stages in the metamorphoses of the beetles. The insects we have mentioned are those with a "complete metamorphosis." We have seen that even in them the term "complete" is a relative and not absolute expression, and that the terms larva and pupa are convenient designations for states varying in duration, and assumed to fulfil certain ends of existence, and even then dependent on length of seasons, variation in climate, and even on the locality. When we descend to the insects with an "incomplete" metamorphosis, as in the May fly, we find that, as in the case of Chloëon, Sir John Lubbock has described twenty-one stages of existence, and let him who can say where the larval ends and the pupal or imaginal stages begin. So in a stronger sense with the grasshopper and cockroach. The adult state in these insects is attained after a number of moults of the skin, during each of which the insect gradually draws nearer to the final winged form. But even the so-called pupæ, or half winged individuals known not to be adult, in some cases feel the sexual impulse, while a number of species in each of the families represented by these two insects never acquire wings.

205. Larva. 206. Semi-pupa.
207. Advanced Semi-pupa. 208. Pupa.

EARLY STAGES OF THE HUMBLE BEE.

Still how did the perfect metamorphosis arise? We can only answer this indirectly by pointing to the Panorpa and Caddis flies, with their nearly perfect metamorphosis, though more nearly allied otherwise to those Neuroptera with an incomplete metamorphosis, as the lace-winged fly, than the insects of any other suborder. If, among a group of insects such as the Neuroptera, we find different families with all grades of perfection in metamorphosis, it is possible that larger and higher groups may exist in which these modes of metamorphosis may be fixed and characteristic of each. Had we more space for the exposition of many known facts, the sceptic might perceive that by observing how arbitrary and dependent on the habits of the insects are the metamorphoses of some groups, the fixed modes of other and more general groups may be seen to be probably due to biological causes, or in other words have been acquired through changes of habits or of the temperature of the seasons and of climates. Many facts crowd upon us, which might serve as illustrations and proofs of the position we have taken. For instance, though we have in tropics rainy and dry seasons when, in the latter, insects remain quiescent in the chrysalis state as in the temperate and frigid zones, yet did not the change from the earlier ages of the globe, when the temperature of the earth was nearly the same the world over, to the times of the present distribution of heat and cold in zones, possibly have its influence on the metamorphoses of insects and other animals? It is a fact that the remains of those insects with a complete metamorphosis (the bees, butterflies and moths, flies and beetles) abound most in the later deposits, while those with an incomplete metamorphosis are fewer in number and the earliest to appear. Again, certain groups of insects are not found in the polar regions. Their absence is evidently due to the adverse climatic conditions of those regions. The development of the same groups is striking in the tropics, where the sum of environing conditions all tend to favor the multiplication of insect forms.

209. Jaws of Ant Lion.

It should be observed that some insects, as the grasshopper, for example, as Müller says, "quit the egg in a form which is distinguished from that of the adult insect almost solely by the want of wings," while the freshly hatched young of the bee, we may add, is farthest from the form of the adult. It is evident that in the young grasshoppers, the metamorphoses have been passed through, so to speak, in the egg, while the bee larva is almost embryonic in its build. The helpless young maggot of the wasp, which is fed solely by the parent, may be compared to the human infant, while the lusty young grasshopper, which immediately on hatching takes to the grass or clover field with all the enthusiasm of a duckling to its native pond, may be likened to that young feathered mariner. The lowest animals, as a rule, are at birth most like the adult. So with the earliest known crustacea. The king crabs, and in all probability the primeval trilobites, passed through their metamorphoses chiefly in the egg. So in the ancient Nebaliads (Peltocaris, Discinocaris and Ceratiocaris), if we may follow the analogy of the recent Nebalia, the young probably closely resembled the adult, while the living crabs and shrimps usually pass through the most marked metamorphoses. Among the worms, the highest, and perhaps the most recent forms, pass through the most remarkable metamorphoses.