The halteres—commonly called balancers or poisers—are perhaps the most characteristic of all the Dipterous structures, though they are absent in most of the few wingless forms of the Order. Outside the Diptera similar organs appear to exist only in male Coccidae. The pair of halteres is placed on the metathorax, one on each of the pleural regions. They are believed to be the homologues of the hind wings; Weinland states[[356]] that certain canals existing in the interior of the halter correspond to wing-nervures. The halter may be described as a small rod-like body with a head like a pin, this terminal part being, however, rather variable in form. We have already stated that in many Diptera the squama forms a hood, the position of which leads to the belief that it is an important adjunct to the halter. Although the exact functions of the halteres are far from clear, it is certain that they are highly complex bodies, of extremely delicate structure: they are doubtless sense-organs, possessing as they do, groups of papillae on the exterior and a chordotonal organ (a structure for assisting the perception of sound) in the basal part; each halter is provided with four muscles at the base, and can, like the wings, execute most rapid vibrations. Seeing that they are the homologues of wings, it is a remarkable fact that in no Diptera are they replaced by wings, or by structures intermediate between these two kinds of organs.
Internal Structure.—Information about the internal anatomy is by no means extensive. The tracheal system is highly developed, and has air-sacs connected with it; a large pair at the base of the abdomen being called aërostats by Dufour. Inside the thoracic spiracles there are peculiar structures supposed by some to be voice-organs, while the abdominal spiracles are said to be remarkably simple in structure. Lowne says that there are ten or eleven pairs of spiracles in the Blow-fly; one of these, near the base of the wing, is peculiar in structure, and may not be a true stigma; he calls it a tympanic spiracle; it seems doubtful whether there are more than seven abdominal pairs. The alimentary canal is very elongate, and is provided with a diverticulum, the crop; this is usually called the sucking stomach, though its function is extremely doubtful. The Malpighian tubes are four in number, and are very elongate; in several groups of Nemocera there are, however, five Malpighian tubes, a number known to occur in only very few other Insects. The nervous system is remarkable on account of the concentration of ganglia in the thorax, so as to form a thoracic, in addition to the usual cephalic, brain. For particulars as to the positions of the ganglia and the great changes that occur in the lifetime, the student should refer to Brandt, to Künckel, and to Brauer.[[357]] Much information as to the internal anatomy of the Blowfly is given by Lowne, but it is doubtful to what extent it is applicable to Diptera in general.[[358]]
Fig. 217—Acephalous larva or maggot of the blow-fly, with the head, a, extended. (After Lowne.)
The larvae of Diptera are—so far as the unaided eye is concerned—without exception destitute of any kind of adornment, the vast majority of them being of the kind known as maggots. None of them have true thoracic legs; though in the earlier groups, pseudopods or protuberances of the body that serve as aids in locomotion are common. Unlike what occurs in other Orders the arrangement of these pseudopods on the body differs greatly in various forms; in a few cases they are surmounted by curved hairs. The most important distinction in external form in Dipterous larvae is that while those that are thorough maggots possess no visible head, others have a well-marked one (Fig. 225); these are therefore called "eucephalous": they have a mouth of the mandibulate type. In some other Dipterous larvae the head is more or less reduced in size, and in the acephalous forms there is only a framework of a few chitinous rods to represent it. The nervous system in the most completely headless larvae is very remarkable, all the ganglia being concentrated in a single mass placed in the thorax. The tracheal system exhibits a great variety; some larvae have stigmata arranged along the sides of the body after the fashion normal in Insect-larvae; these are called "peripneustic"; as many as ten pairs of stigmata may be present in these cases, but nine pairs is much more common. Other larvae have a pair of stigmata placed at the termination of the body, and another pair near the anterior extremity, the two pairs communicating by large tracheal trunks extending the length of the body; these larvae are said to be "amphipneustic": this is the condition usual in the more completely acephalous larvae. Others have only the terminal pair of spiracles, and are styled "metapneustic." Some begin life in the metapneustic state and afterwards become amphipneustic. In the aquatic larva of Corethra there are no spiracles, though there is an imperfect tracheal system. Many Dipterous larvae that live in water or in conditions that prevent access of air to the body have remarkable arrangements for keeping the tip of the body in communication with the atmosphere. The stigmata in metapneustic and amphipneustic larvae are very remarkable compound structures, exhibiting however great diversity; their peculiarities and uses are not well understood; it appears very doubtful whether some of them have any external opening. Reference may be made, as to the variety of structure, to Meijere's paper[[359]] from which we take the accompanying figure of a posterior stigmatic apparatus in Lipara lucens. It appears that there is a compound chamber—"Filzkammer"—terminating externally in lobes or fingers—"Knospen" and appearing as marks on the outer surface: this chamber is seated on a tracheal tube, and is, Meijere thinks, probably a secondary growth of the trachea coming to the outer surface. It is traversed by what may be considered the original tracheal tube, opening externally as an external stigmatic scar—"Stigmennarbe"—and with a second or inner scar placed internally. We may conclude from what is already known that these structures will be found to differ in the same larva according to the stage of its development.
Fig. 218—The posterior stigma of the larva of Lipara lucens. a, One of the three "Knospen" or lobes; b, external stigmatic scar; c, internal scar; d, stigmatic chamber (Filzkammer); e, trachea. (After Meijere.)
An extremely valuable summary of the characters and variety of Dipterous larvae has been given by Brauer,[[360]] from which it appears that the larvae of the first half of the family exhibit great variety and have been much studied, while the more purely maggot-like forms of the Muscidae have, with one or two exceptions, been little investigated.
The pupal instar is of two distinct kinds. First, we meet with a pupa like that of Lepidoptera, viz. a mummy-like object, or pupa obtecta, in which there is a crisp outer shell, formed in part by the adherent cases of the appendages of the future imago. This condition, with a few exceptions to be subsequently noticed, obtains in the Nemocera and Brachycera. It is exhibited in various degrees of perfection, being most complete in Tipulidae; in other forms the shell is softer and the appendages more protuberant. The second kind of pupa is found in the Cyclorrhaphous flies; it has externally no marks except some faint circular rings and, frequently, a pair of projections from near one extremity of the body; occasionally there is a single prominence at the other extremity of the body. This condition is due to the fact that the larva does not escape from the skin at the last ecdysis, but merely shrinks within it, so that the larval skin, itself contracted and altered by an excretion of chitin, remains and forms a perfect protection to the included organism. This kind of pupa looks like a seed, and is well exemplified by the common Blow-fly. The capacity for entering on such a condition is evidently correlative with the absence of a larval head. The metamorphosis in this curious little barrel goes on in a different manner to what it does in the pupa obtecta. A good name for the whole structure of this instar has not been found. Older authors called it "pupa coarctata," or "nympha inclusa"; Brauer speaks of it as a "compound pupa"; ordinarily in our language it is called a "puparium," a term which is more applicable to the case alone.
In species having a pupa obtecta the larval skin is cast after the chief processes of the external metamorphosis have occurred, and then an exudation of chitin hardens the general surface. In the "compound pupa" of the Blow-fly there is for a considerable period no formed pupa at all, but merely a shell or case containing the results of histolysis and the centres for regeneration of new organs; the chitin-exudation to the exterior of the larval skin occurs in the early part of the series of metamorphic changes, and the organism breaks down to a cream within the shell thus formed, and then gradually assumes therein the condition of a soft, nymphoid pupa. The exceptional conditions previously referred to as exhibited by a few forms are certain cases in which a more or less perfect pupa obtecta is found within the last larval skin, as is the case in Stratiomys. Another highly remarkable condition exists in the Hessian fly, and a few other Cecidomyiids, where the Insect apparently makes an exudation which it uses as a covering case, independent of the larval skin; this latter being subsequently shed inside the case, so that this condition of coarctate pupa differs from that we have described as existing in Cyclorrhaphous flies, although the two are superficially similar. In the Pupipara the larval stage is passed in the body of the mother, which produces a succession of young, nourished one at a time by the secretion of glands; this young is born as a full-grown larva that becomes at once a pupa.