Fam. 32. Syrphidae (Hover-flies).—Of moderate or rather large size, frequently spotted or banded with yellow, with a thick fleshy proboscis capable of being withdrawn into a cleft on the under side of the head; antennae not placed in definite cavities, three-jointed (usually very short), and leaving a seta that is not terminal in position, and may be feathered. Squama variable, never entirely covering the halteres; the chief (third to fifth) longitudinal veins of the wings connected near their termination by cross-veins and usually thus forming a sort of short margin parallel with the hind edge of the wing; a more or less imperfect false nervure running between the third and fourth longitudinal nervures; no empodium and generally no distinct system of bristles on the back of the body. The Syrphidae (Fig. 212) form one of the largest and best known of all the families of flies; they abound in our gardens where, in sunny weather, some species may be nearly always seen hovering over flowers, or beneath trees in places where the rays of the sun penetrate amidst the shade. There are two or three thousand species known, so that of course much variety exists; some are densely covered with hair (certain Volucella and others), many are of elegant form, and some bear a considerable resemblance to Hymenoptera of various groups. The peculiar veining of the wings permits of their easy identification, the line of two nervules, approximately parallel with the margin of the distal part of the wing (Fig. 212, D), and followed by a deep bay, being eminently characteristic, though there are some exceptions; there are a few forms in which the antennae are exceptional in having a terminal pointed process. The proboscis, besides the membranous and fleshy lips, consists of a series of pointed slender lancets, the use of which it is difficult to comprehend, as the Insects are not known to pierce either animals or vegetables, their food being chiefly pollen; honey is also doubtless taken by some species, but the lancet-like organs appear equally ill-adapted for dealing with it. The larvae are singularly diversified; first, there are the eaters of Aphidae, or green-fly; some of these may be generally found on our rose-bushes or on thistles, when they are much covered with Aphids; they are soft, maggot-like creatures with a great capacity for changing their shape and with much power of movement, especially of the anterior part of the body, which is stretched out and moved about to obtain and spear their prey: some of them are very transparent, so that the movements of the internal organs and their vivid colours can readily be seen: like so many other carnivorous Insects, their voracity appears to be insatiable. The larvae of many of the ordinary Hover-flies are of this kind. Eristalis and its allies are totally different, they live in water saturated with filth, or with decaying vegetable matter (the writer has found many hundreds of the larvae of Myiatropa florea in a pool of water standing in a hollow beech-tree). These rat-tailed maggots are of great interest, but as they have been described in almost every work on entomology, and as Professor Miall[[422]] has recently given an excellent account of their peculiarities, we need not now discuss them. Some of the flies of the genus Eristalis are very like honey-bees, and appear in old times to have been confounded with them; indeed, Osten Sacken thinks this resemblance gave rise to the "Bugonia myth," a fable of very ancient origin to the effect that Honey-bees could be procured from filth, or even putrefying carcases, by the aid of certain proceedings that savoured slightly of witchcraft, and may therefore have increased the belief of the operator in the possibility of a favourable result. It was certainly not bees that were produced from the carcases, but Osten Sacken suggests that Eristalis-flies may have been bred therein.
In the genus Volucella we meet with a third kind of Syrphid larva. These larvae are pallid, broad and fleshy, surrounded by numerous angular, somewhat spinose, outgrowths of the body; and have behind a pair of combined stigmata, in the neighbourhood of which the outgrowths are somewhat larger; these larvae live in the nests of Bees and Wasps, in which they are abundant. Some of the Volucella, like many other Syrphidae, bear a considerable resemblance to Bees or Wasps, and this has given rise to a modern fable about them that appears to have no more legitimate basis of fact than the ancient Bees-born-of-carcases myth. It was formerly assumed that the Volucella-larvae lived on the larvae of the Bees, and that the parent flies were providentially endowed with a bee-like appearance that they might obtain entrance into the Bees' nests without being detected, and then carry out their nefarious intention of laying eggs that would hatch into larvae and subsequently destroy the larvae of the Bees. Some hard-hearted critic remarked that it was easy to understand that providence should display so great a solicitude for the welfare of the Volucella, but that it was difficult to comprehend how it could be, at the same time, so totally indifferent to the welfare of the Bees. More recently the tale has been revived and cited as an instance of the value of deceptive resemblance resulting from the action of natural selection, without reference to providence. There are, however, no facts to support any theory on the subject. Very little indeed is actually known as to the habits of Volucella in either the larval or imaginal instars; but the little that is known tends to the view that the presence of the Volucella in the nests is advantageous to both Fly and Bee. Nicolas has seen Volucella zonaria enter the nest of a Wasp; it settled at a little distance and walked in without any fuss being made. Erné has watched the Volucella-larvae in the nests, and he thinks that they eat the waste or dejections of the larvae. The writer kept under observation Volucella-larvae and portions of the cells of Bombus, containing some larvae and pupae of the Bees and some honey, but the fly-larvae did not during some weeks touch any of the Bees or honey, and ultimately died, presumably of starvation. Subsequently, he experimented with Volucella-larvae and a portion of the comb of wasps containing pupae, and again found that the flies did not attack the Hymenoptera; but on breaking a pupa of the Wasp in two, the fly-larvae attacked it immediately and eagerly; so that the evidence goes to show that the Volucella-larvae act as scavengers in the nests of the Hymenoptera. Künckel d'Herculais has published an elaborate work on the European Volucella; it is remarkable for the beauty of the plates illustrating the structure, anatomy and development, but throws little direct light on the natural history of the Insects. V. bombylans, one of the most abundant of our British species, appears in two forms, each of which has a considerable resemblance to a Bombus, and it has been supposed that each of the two forms is specially connected with the Bee it resembles, but there is no evidence to support this idea; indeed, there is some little evidence to the contrary. The genus Merodon has larvae somewhat similar to those of Volucella, but they live in bulbs of Narcissus; M. equestris has been the cause of much loss to the growers of Dutch bulbs; this Fly is interesting on account of its great variation in colour; it has been described as a whole series of distinct species.
The most remarkable of the numerous forms of Syrphid larvae are those of the genus Microdon (Fig. 239), which live in ants' nests. They have no resemblance to Insect-larvae, and when first discovered were not only supposed to be little Molluscs, but were actually described as such under the generic names of Parmula and Scutelligera. There is no appearance of segmentation of the body; the upper surface is covered by a sort of network formed by curved setae, which help to retain a coating of dirt; there is no trace externally of any head, but on the under surface there is a minute fold in which such mouth-organs as may be present are probably concealed; the sides of the body project so as to form a complex fringing arrangement; the terminal stigmata are very distinct, the lateral processes connected with them (the "Knospen" of Dr. Meijere), are, however, very irregular and placed at some distance from the stigmatic scar. Pupation occurs by the induration of the external covering and the growth from it, or rather through it, of two short horns in front. Inside this skin there is formed a soft pupa, of the kind usual in Cyclorrhaphous flies; the dehiscence of the external covering is, however, of unusual nature, three little pieces being separated from the anterior part of the upper surface, while the lower face remains intact. The account of the pupation given by Elditt[[423]] is not complete: the two horns that project are, it would appear, not portions of the larval skin, but belong to the head of the pupa, and according to Elditt are used to effect the dehiscence of the case for the escape of the fly; there does not appear to be any head-vesicle. Nothing is known as to the details of the life of these anomalous larvae. M. Poujade has described two species found in France in the nests of the ant Lasius niger.[[424]] The larva we figure was found by Colonel Yerbury in nests of an Atta in Portugal, and an almost identical larva was recently found by Mr. Budgett in Paraguay. The flies themselves are scarce, Microdon mutabilis (formerly called M. apiformis) being one of the rarest of British flies. They have the antennae longer than is usual in Syrphidae, and the cross-veins at the outside of the wing are irregularly placed, so that the contour is very irregular: the resemblance to bees is very marked, and in some of the South American forms the hind legs are flattened and hairy like those of bees. The oviposition of Microdon has been observed by Verhoeff;[[425]] he noticed that the fly was frequently driven away by the ants—in this case, Formica sanguinea—but returned undiscouraged to its task.
Fig. 239—Larva of Microdon sp. Portugal. A, Dorsal view of the larva, × 4; 1, the stigmatic structure; B, posterior view of stigmatic structure; C, a portion of the marginal fringe of the body.
A brief résumé of the diverse modes of life of Syrphid larvae has been given by Perris,[[426]] and he also gives some information as to the curious horns of the pupae, but this latter point much wants elucidation. Whether the Syrphidae, or some of them, possess a ptilinum that helps them to emerge from the pupa is more than doubtful, though its existence has been affirmed by several authors of good repute.[[427]]
Fig. 240.—Diopsis apicalis. Natal. A, The fly; B, extremity of cephalic protuberance, more magnified. a, The eye; b, the antenna; C, middle of head, front view; c, ocelli.
Series 4. Cyclorrhapha Schizophora
Fam. 33. Muscidae acalyptratae.—This group of flies has been the least studied of all the Diptera; it is generally treated as composed of twenty or thirty different families distinguished by very slight characters. It is, however, generally admitted by systematists that these assemblages have not the value of the families of the other divisions of Diptera, and some even go so far as to say that they are altogether only equivalent to a single family. We do not therefore think it necessary to define each one seriatim; we shall merely mention their names, and allude to certain points of interest connected with them. Taken collectively they may be defined as very small flies, with three-jointed antennae (frequently looking as if only two-jointed), bearing a bristle that is not terminally placed; frequently either destitute of squamae or having these imperfectly developed so as not to cover the halteres; and possessing a comparatively simple system of nervuration, the chief nervures being nearly straight, so that consequently few cells are formed. These characters will distinguish the group from all the other Diptera except from forms of Aschiza, and from certain Anthomyiidae, with both of which the Acalyptratae are really intimately connected. Considerable difference of opinion prevails as to the number of these divisions, but the families usually recognised are:—