Crossopterygii

“ The independent evolution of a more specialised vertebral column from a more primitive one may often be traced within the limits of the same group of Fishes when the more ancient genera are compared with the more recent. In the Elasmobranchs and the Crossopterygii, for example, the oldest known types were acentrous, while the more recent have acquired calcified or bony centra, and altogether they have reached a more advanced stage of vertebral evolution. ”

“ Now, the majority of the Orthoptera quit the egg in a form which is distinguished from that of the adult Insect almost solely by the want of wings; these larvæ then soon acquire rudiments of wings, which appear more strongly developed after every moult. Even this perfectly gradual transition from the youngest larva to the sexually mature Insect, preserves in a far higher degree the picture of an original mode of development, than does the so-called complete metamorphosis of the Coleoptera, Lepidoptera, or Diptera, with its abruptly separated larva-, pupa- and imago-states. ”

“ 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 {130}first division, or Heteromorphæ as they are called; and the Hemiptera and Orthoptera into the second or Homomorphæ. ”

“ If we admit that the other orders of insects, the Lepidoptera, the Hemiptera, the Hymenoptera, the Neuroptera, the Orthoptera, the Diptera, the Strepsiptera, comprise, taken altogether, at least the same number of species as the Coleoptera alone, we shall gain, for the class of insects, a total of 200,000. And we shall certainly keep within the truth if we assign the same number of species to the Annelida, the Crustacea, the Arachnida, the Myriapoda, and the Monomorpha, to which, with some modification, we may apply the remarks already called forth by the Coleoptera. ”

“ The development of a typical fish mouth is the next step in evolution, and with its appearance we note the decline of the air-bladder in size and function.
The Crossopterygians.—The fish-like mouth appears with the group of Crossopterygians, fishes which still retain the old-fashioned type of pectoral and ventral fin, the archipterygium. In the archaic tail, enameled scales, and cartilaginous skeleton the Crossopterygian shows its affinity with its Dipnoan ancestry. Thus these fishes unite in themselves traits of the shark, lung-fish, and Ganoid. ”

“ The circumstance that there are still numerous wingless species among the Orthoptera, and that some of these (Blattidæ) are so like certain Crustacea (Isopods) in habit that both are indicated by the same name (“Baratta”) by the people in this country, can scarcely be regarded as of any importance.
The contrary supposition that the oldest Insects possessed a “complete metamorphosis,” and that the “incomplete metamorphosis” of the Orthoptera and Hemiptera is only of later origin, is met by serious difficulties. ”

“ The Coleoptera, the Diptera, or the Hymenoptera, on the other hand, present far greater and more essential variations. In either of these orders we have both vegetable- and animal-feeders, aquatic, and terrestrial, and parasitic groups. Whole families are devoted to special departments in the economy of nature. Seeds, fruits, bones, carcases, excrement, bark, have each their special and dependent insect tribes from among them; whereas the Lepidoptera are, with but few exceptions, confined to the one function of devouring the foliage of living vegetation. ”

“ The fact that in a majority of instances it is possible to trace an intimate connection between the surroundings of a larva and its organization proves in the clearest way that the characters of the majority of existing larval forms of insects have owed their origin to secondary adaptations. A few instances will illustrate this point:—In the simplest types of metamorphosis, e.g. those of the Orthoptera genuina, the larva has precisely the same habits as the adult. We find that a caterpillar form is assumed by phytophagous larvæ amongst the Lepidoptera, Hymenoptera, and Coleoptera. ”

“ But nowhere among the Crustacea is there a mode of development comparable to the “complete metamorphosis” of the Insecta, nowhere among the young or adult Crustacea are there forms which might resemble the maggots of the Diptera or Hymenoptera, the larvæ of the Coleoptera, or the caterpillars of the Lepidoptera, still less any bearing even a distant resemblance to the quiescent pupæ of these animals. ”

“ The foregoing brief outline of our knowledge of the geological succession of insects shows that the exopterygote preceded, in time, the endopterygote type of life-history. We have already seen that those insects undergoing little change in the life-cycle, and with visible, external wing-rudiments, are on the whole less specialised in structure than those which pass through a complete transformation. These two considerations, taken together, suggest strongly that in the evolution of the insect class, the simpler life-history preceded the more complex. ”

“ I wish it to be observed, that Neuroptera, in the genera Psyche, Cloëon, Termes, Psocus and Mantispa, does not merely assume the form of the genera, Tinea, Chironomus, Formica, Aphis and Mantis, but actually possesses the characters and appearance of the classes Lepidoptera, Diptera, Hymenoptera, Hemiptera and Orthoptera. The obviously homogeneous character of Lepidoptera and Coleoptera, although, probably, containing in every sub-class more species than the whole of Neuroptera, clearly disproves the existence of variety amongst their contents, equal to that amongst the contents of Neuroptera. ”

“ It deserves notice, that in the class Crustacea, both in the Lerneidæ and in the Cirripedia, the males more closely resemble the larvæ, than do the females; whereas amongst insects, as in the case of the glow-worm in Coleoptera, and of certain nocturnal Lepidoptera, it is the female which retains an embryonic character, being worm-like or caterpillar-like, without wings. But in all these cases, the male is more locomotive than the female. ”

“ As shown by the number and variety of species, the Orthoptera are the most dominant order of this group. Eminently terrestrial in habit, the differentiation of their fore-wings and hind-wings can be traced from Carboniferous, isopteroid ancestors through intermediate Mesozoic forms. The Plecoptera resemble the Ephemeroptera and Odonata in the aquatic habits of their larvae, and by the occasional presence of tufted thoracic gills in the imago exhibit an aquatic character unknown in any other winged insects. ”

“ Those in which the adults differ from the freshly hatched young only or mainly in having wings are generally said to have an incomplete or gradual metamorphosis. There is no inactive, resting, or pupal stage, and the wings are acquired only after successive moults. Insects with an incomplete metamorphosis are the Orthoptera, Dermaptera, Platyptera (Mallophaga, Plecoptera, Corrodentia, Embidæ) , Ephemeridæ, Odonata, Thysanoptera, and Hemiptera, with the exception of the male Coccidæ, in which there is a resting or sub-nymph stage. ”

“ They all have feet, and the body is more or less flattened. (Fig. 560.)
II. Holometabola.—In this division we have for the first time a true larva, and a pupa stage as distinguished from the imago. Moreover, the insect at each stage is distinguished by radical differences in form, surroundings, and in the nature of the food, while the pupa is inactive, usually immovable, and incapable of taking any food, and is often protected by a cocoon spun by the larva. The holometabolous orders are the Neuroptera, Coleoptera, Mecoptera, Trichoptera, Lepidoptera, Siphonaptera, Diptera, and Hymenoptera. ”

“ The earliest, most primitive pterygote insects passed through only a slight metamorphosis. In other words, as soon as the wings were evolved and insects became adapted to live or take refuge in a new medium, the air, at the approach of the period of adult life, with the ripening or perfection of the reproductive organs, a metamorphosis began to take place, and the number of species greatly multiplied. On the other hand, the Arachnida and Myriopoda, in which as a rule there is no metamorphosis, being confined to a creeping life, with no change of medium, remained poor in number of species. ”

“ All insects do not follow the same method of development from egg to adult, and the adaptations of structure and habit are many and varied as well as simple and complex. Species having a complex development, during which they pass through stages, each differing in form from its predecessor, undergo what is known as a metamorphosis; contrasted with such insects are those developing in a simple manner without pronounced differences in the form of successive stages, the young resembling the adult in most features except size and maturity—​these insects are without a metamorphosis. ”

“ The same may be said of the true Neuroptera, Trichoptera (Fig. 157) , the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly, [27] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps; [28] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum. ”

“ 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. ”

“ A proof that this greater sensibility to modifying causes is not imaginary may, I think, be drawn from the consideration that while the Lepidoptera as a whole are of all insects the least essentially varied in form, structure, or habits, yet in the number of their specific forms they are not much inferior to those orders which range over a much wider field of nature, and exhibit more deeply seated structural modifications. The Lepidoptera are all vegetable-feeders in their larva-state, and suckers of juices or other liquids in their perfect form. ”

“ In conjunction with other evidence it would appear to indicate that the Lepidoptera originated from insects with active pupæ. The number of free or movable segments of pupæ varies considerably in different groups and genera, and by some modern authors it is regarded as a character of much importance in the framing of their classifications. The various organs of the perfect insect are distinctly marked out on the otherwise uniform integument of the pupa. In some groups, notably the Micropterygina, these organs are much more distinctly indicated than in others. ”

“ The Neuroptera, with their similar fore- and hind-wings and their campodeiform larvae, seem to stand nearest to the presumed isopteroid ancestry, but the imago and larva are often specialized. The campodeiform larvae of many Coleoptera are indeed far more primitive than the neuropteran larvae, and suggest to us that the Coleoptera—modified as their wing-structure has become—arose very early from the primitive metabolous stock. ”

“ Sometimes all the four are of similar membranous structure, and either all equal in size, (Neuroptera,) or the anterior pair largest, (Hymenoptera, &c.) At other times, the anterior are rigid and horny throughout their whole extent, (Coleoptera,) or only on one-half of their surface, (Hemiptera Heteroptera.) In some instances, (Orthoptera, &c.) the anterior wings are what is called pergameneous—that is, of a substance between coriaceous and membranous, or somewhat resembling parchment.
The harder the substance of which wings are composed, the less active is the share they take in flight. ”

“ When the pupal stage is complete the insect has to make its way out of the cocoon. In the lower families of moths it is the pupa which comes out at least partially, working itself onwards by the spines on its abdominal segments; the pupa of the primitive Micropteryx has functional mandibles with which it bites through the cocoon. In the higher Lepidoptera the pupa is immovable, and the imago, after the ecdysis of the pupal cuticle, must emerge. ”

“ In regard to number they vary extremely; two is the prevailing number among the Coleoptera, Hemiptera, and Diptera, three occur among some coleopterous tribes, such as the Cerambycidæ, four in many Diptera and a few Neuroptera, six in the Lepidoptera, eight in certain neuropterous groups, such as the Hemerobii, and, in many instances, particularly among the Orthoptera and Hymenoptera, they amount to a hundred and fifty. Their length generally bears some proportion to their number, a deficiency the one way being compensated by an increase in the other. ”