Metamorphosis.—As it is in Diptera that the phenomena of Insect-metamorphosis have reached their highest development we endeavoured to give some idea of their nature in the previous volume, therefore we need give only a brief sketch of the chief features of Dipterous metamorphosis. The Blow-fly undergoes a rapid embryonic development, the later stages of which are, on the whole, of a retrogressive nature. On the emergence of the young maggot it feeds up rapidly, the rapidity varying greatly according to circumstances, and then when full-grown rests. While resting, a process of internal liquefaction, called histolysis, is going on, and the maggot contracts and exudes an excretion that hardens its skin. At the time this hard skin has become complete, or soon after, the maggot inside has dissolved into a cream contained in a sac inside the shell; this cream becomes reconstituted into a fly by a gradual process of growth and development of certain minute portions of the body—the imaginal discs or folds, the histoblasts and neuroblasts that were exempt from the histolytic process: in the early stages of the reconstitution the general structure is, of course, altogether vague, and this condition—purely one of transition—is called the pronymph; the nymph becomes gradually developed: it corresponds vaguely with the pupa obtecta of the early groups of Diptera, but is soft like the pupa of Hymenoptera. This nymph gradually develops into the fly itself, the external parts being first completed and the internal organs elaborated subsequently. The sexual organs do not undergo metamorphosis like other internal organs, there being a gradual (though irregular or interrupted) growth of them in the young larva, till they are completed some time after the emergence of the perfect fly. The processes in the Blow-fly have been studied by numerous able histologists of various nationalities, and have recently been described by Lowne in our own language.[[361]] Comparatively little has been done in studying the corresponding phenomena in other Diptera. Weismann has investigated the development of Corethra, and Miall that of Chironomus. These two flies belong to a division of Diptera different from that which includes the Blow-fly, and they display a condition of the metamorphic processes allied to what occurs in Lepidoptera, as well as to that which takes place in the Blow-fly. Imaginal folds are formed, but they only appear much later in the life, and they are much less distant from the positions they will, when developed, occupy in the imago. In Chironomus, according to Miall, the imaginal folds only appear in the last larval instar, but they grow with such rapidity that the legs and wings of the future fly can be distinguished in the larva, even before pupation; thus when the activity of the larva ceases but little change is required to complete the obtected pupa. In the Blow-fly some of the imaginal folds have been traced back to the embryo; how many centres for the new growth there may be is uncertain, for though there are upwards of sixty for the outer body, the number of regenerative centres for the internal organs is not ascertained. The peculiar central nervous mass, mentioned in our remarks on the larva, consists of two kinds of tissue mixed together in a complex manner; one of these kinds is functionally active during the larval life and at the metamorphosis undergoes histolysis, while the other, or embryonic, portion develops into the nervous system of the fly.
It forms no part of our task to deal with general subjects, but we may be pardoned for calling attention to the bearing the metamorphosis of the higher Diptera has on our ideas of heredity in Insects. The fly bears no resemblance whatever to the larva, and is only obtained by the organic destruction of the latter, which occurs before the perfection of the sexual organs takes place, and yet the fly reproduces itself only secondarily, but primarily gives rise to the totally different larva. It is supposed that the larval structures have been gradually acquired, and yet they are transmitted with the utmost faithfulness by the totally different fly. We can only conclude that that which is bequeathed in each species is the early state of a particular process of development from which the subsequent stages follow necessarily if the developing organism be placed in conditions having on it influences like to those that influenced the ancestors.
Classification.—The classification of Diptera is as yet very imperfect. Formerly they were divided into two great groups, Nemocera and Brachycera, according to the structure of the antennae, as previously mentioned. This division has been abandoned, and the term Brachycera is now applied to only a small part of the old section that bore the name. The primary division usually adopted at present is into Orthorrhapha and Cyclorrhapha. The characters of these two groups are based on the nature of the metamorphosis, and have been gradually elaborated by Brauer in various memoirs.[[362]] The Orthorrhapha includes the forms with obtected pupae, the Cyclorrhapha those with a nymph-compound, as previously described. This distinction is of great importance, but unfortunately it is difficult to apply to the fly itself; the only character that can be used in connection with the imago is the existence of a suture over the insertion of the antennae in a portion, but not all, of the Cyclorrhapha.[[363]] The next set of divisions used by Brauer divides the Order into four sections, viz. 1. Orthorrhapha Nematocera, 2. O. Brachycera, 3. Cyclorrhapha Aschiza, 4. C. Schizophora. As these four groups are recognised more readily than the two major groups the student will do well at first to disregard the primary division and consider the Diptera as divisible into four great groups. To these four divisions we, however, add temporarily a fifth, viz. Pupipara. This is included by Brauer in Schizophora, but it appears to be really an unnatural complex, and had better be kept separate till it has been entirely reconsidered. These great sections may be thus summarised:—
Series 1. Orthorrhapha Nemocera.—Antennae with more than 6 segments, not terminated by an arista; with the segments of the flagellum more or less similar to one another. Palpi slender and flexible, four- or five-jointed.[[364]]
Series 2. Orthorrhapha Brachycera.—Antennae variable, but never truly Nemocerous nor like those of Cyclorrhapha; when an arista is present it is usually placed terminally, not superiorly; when an arista is not present the flagellum terminates as an appendage consisting of a variable number of indistinctly separated segments; thus the flagellum is not composed of similar joints; [rarely are the antennae as many as seven-jointed]. Palpi only one- or two-jointed.[[365]] Around the insertion of the antennae there is no definite arched suture enclosing a small depressed space. The nervuration of the wings is usually more complex than in any of the other divisions.
Series 3. Cyclorrhapha Aschiza.—Antennae composed of not more than three joints and an arista; the latter is not terminal. Front of head without definite arched suture over the antennae, but frequently with a minute area of different colour or texture there. This group consists of the great family Syrphidae, and of four small families, viz. Conopidae, Pipunculidae, Phoridae, and Platypezidae. The section is supposed to be justified by its being Cyclorrhaphous in pupation, and by the members not possessing a ptilinum (or having no trace of one when quite mature). The Syrphidae are doubtless a natural group, but the association with them of the other families mentioned is a mere temporary device. The greatest difficulty is experienced in deciding on a position for Phoridae, as to which scarcely any two authorities are agreed.
Series 4. Cyclorrhapha Schizophora, or Eumyiid flies. The antennae consist of three joints and an arista. In the Calyptratae the frontal suture, or fold over the antennae, is well marked and extends downwards along each side of the face, leaving a distinct lunule over the antennae. In the Acalyptrate Muscids the form of the head and of the antennae vary much and are less characteristic, but the wings differ from those of Brachycera by their much less complex nervuration.
Series 5. Pupipara. These are flies of abnormal habits, and only found in connection with living Vertebrates, of which they suck the blood (one species, Braula caeca, lives on bees). Many are wingless, or have wings reduced in size. The young are produced alive, full grown, but having still to undergo a metamorphosis. This group consists of a small number of flies of which some are amongst the most aberrant known. This is specially the case with the Nycteribiidae. This Section will probably be greatly modified, as it is far from being a natural assemblage.[[366]]
The Sub-Order Aphaniptera, or Fleas, considered a distinct Order by many entomologists, may for the present be placed as a part of Diptera.
It must be admitted that these sections are far from satisfactory. Brauer divides them into Tribes, based on the nature of the larvae, but these tribes are even more unsatisfactory than the sections, hosts of species being entirely unknown in the larval state, and many of those that are known having been very inadequately studied. We must admit that the classification of Diptera has at present advanced but little beyond the stage of arranging them in natural families capable of exact definition. We may, however, draw attention to the attempt that is being made by Osten Sacken to remodel the classification of the Nemocera and Brachycera by the combination of families into super-families.[[367]] He proposes to divide the Nemocera into two super-families: 1. Nemocera Vera, including all the families from Cecidomyiidae to Tipulidae; 2. Nemocera Anomala, consisting of the small families Bibionidae, Simuliidae, Blepharoceridae, Rhyphidae and Orphnephilidae.