Fig. 232—Argyromoeba trifasciata. A, Young larva; B, adult larva; C, pupa. France. (After Fabre.) A, Greatly, B, C, slightly magnified.
Fam. 20. Bombyliidae.—Body frequently fringed with down, or covered in large part with hair. Legs slender, claws small, without distinct empodium, usually with only minute pulvilli. Proboscis very long or moderate, antennae three-jointed, terminal joint not distinctly divided, sometimes large, sometimes hair-like. This is a very large family, including 1500 species, and is of great importance to both naturalist and economist. Two well-marked types, formerly treated as distinct families, are included in it—(1) the Bombyliides with very long exserted rostrum, and humped thorax; and (2) Anthracides, with a short beak, and of more slender and graceful form. None of these flies are bloodsuckers, they frequent flowers only, and use their long rostrums in a harmless manner. The members of both of these groups usually have the wings ornamented with a pattern, which in Anthrax is frequently very remarkable; in both, the clothing of the body is frequently variegated. Their powers of flight are very great, and the hovering Bombylius of early spring is endowed with an unsurpassed capacity for movement, remaining perfectly still on the wing, and darting off with lightning rapidity; Anthrax is also most rapid on the wing. In Britain we have but few species of Bombyliidae, but in warm and dry climates they are very numerous. The life-history of these Insects was till recently unknown, but that of Argyromoeba (Anthrax) trifasciata has been described by the French naturalist, Fabre, who ascertained that the species is parasitic on the Mason-bee, Chalicodoma muraria, that forms nests of solid masonry. He endeavoured to discover the egg, but failed; the parent-fly oviposits, it appears, by merely dropping a minute egg while flying over the surface of the mass of masonry by which the grubs of the Chalicodoma are protected. From this egg there is hatched a minute delicate vermiform larva (Fig. 232, A). In order to obtain its food, it is necessary for this feeble creature to penetrate the masonry; apparently a hopeless task, the animal being scarcely a twentieth of an inch long and very slender; it is, however, provided with a deflexed horny head, armed in front with some stiff bristles, while on the under surface of the body there are four pairs of elongate setae serving as organs of locomotion; thus endowed, the frail creature hunts about the surface of the masonry, seeking to find an entrance; frequently it is a long time before it is successful; but though it has never taken any food it is possessed of great powers of endurance. Usually, after being disclosed from the egg, it remains about fifteen days without stirring; and even after it commences its attempts to enter the nest it is still capable of a long life without taking any food. Possibly its organisation will not permit it to feed (supposing any food were obtainable by it) without its growing somewhat thereafter, and if so, its chance of obtaining entrance through the masonry would be diminished. Abstention, it would appear, is the best policy, whether inevitable or not; so the starving little larva continues its endeavours to find a chink of entrance to the food contained in the interior of the masonry. It has plenty of time for this, because it is better for it not to get into the cell of the bee until the grub is quite full grown, and is about to assume the pupal form, when it is quite incapable of self-defence. Finally, after greater or less delay, the persevering little larva succeeds in finding some tiny gap in the masonry through which it can force itself. M. Fabre says that the root of a plant is not more persistent in descending into the soil that is to support it than is this little Anthrax in insinuating itself through some crack that may admit it to its food. Having once effected an entrance the organisation that has enabled it to do so is useless; this primary form of the larva has, in fact, as its sole object to enable the creature to penetrate to its food. Having penetrated, it undergoes a complete change of form, and appears as a creature specially fitted for feeding on the quiescent larva of the bee without destroying it. To accomplish this requires an extreme delicacy of organisation and instinct; to bite the prey would be to kill it, and if this were done, the Anthrax would, Fabre supposes, ensure its own death, for it cannot feed on the dead and putrefying grub; accordingly, the part of its body that does duty as a mouth is merely a delicate sucker which it applies to the skin of the Chalicodoma-grub; and thus without inflicting any perceptible wound it sucks day after day, changing its position frequently, until it has completely emptied the pupa of its contents, nothing being left but the skin. Although this is accomplished without any wound being inflicted, so effectual is the process that all the Chalicodoma is gradually absorbed. The time requisite for completely emptying the victim is from twelve to fifteen days; at the end of this time the Anthrax-larva is full grown, and the question arises, how is it to escape from the cell of solid masonry in which it is imprisoned? It entered this cell as a tiny, slender worm through a minute orifice or crack, but it has now much increased in size, and exit for a creature of its organisation is not possible. For some months it remains a quiescent larva in the cell of the Chalicodoma, but in the spring of the succeeding year it undergoes another metamorphosis, and appears as a pupa provided with a formidable apparatus for breaking down the masonry by which it is imprisoned. The head is large and covered in front with six hard spines, to be used in striking and piercing the masonry, while the other extremity of the body bears some curious horns, the middle segments being armed with rigid hairs directed backwards, and thus facilitating movement in a forward direction and preventing slipping backwards. The pupa is strongly curved, and fixes itself by the aid of the posterior spines; then, unbending itself, it strikes with the armour of the other extremity against the opposing wall, which is thus destroyed piecemeal until a gallery of exit is formed; when this is completed the pupa-skin bursts and the perfect fly emerges, leaving the pupa-case still fixed in the gallery. Thus this species appears in four consecutive forms—in addition to the egg—each of which is highly specialised for the purposes of existence in that stage.
The habits of our British Bombylius major have been partially observed by Dr. Chapman,[[407]] and exhibit a close analogy with those of Anthrax trifasciata. The bee-larva that served as food was in this case Andrena labialis, and the egg was deposited by the fly, when hovering, by jerking it against the bank in which the nest of the bee was placed.
It has recently been discovered that the larvae of various species of Bombyliidae are of great service by devouring the eggs of locusts. Riley found that the egg-cases of Caloptenus spretus are emptied of their contents by the larvae of Systoechus oreas and Triodites mus. A similar observation has been made in the Troad by Mr. Calvert, who found that the Bombyliid, Callostoma fascipennis, destroys large quantities of the eggs of Caloptenus italicus. Still more recently M. Künckel d'Herculais has discovered that the destructive locust Stauronotus maroccanus is kept in check in Algeria in a similar manner, as many as 80 per cent of the eggs of the locust being thus destroyed in certain localities. He observes that the larva of the fly, after being full fed in the autumn, passes the winter in a state of lethargy—he calls it "hypnody"—in the egg-case of the locust, and he further informs us that in the case of Anthrax fenestralis, which devours the eggs of the large Ocnerodes, the lethargy may be prolonged for a period of three years. After the pupa is formed it works a way out of the case by means of its armature, and then again becomes for some days immobile before the perfect fly appears. Lepidopterous larvae are also attacked by Bombyliid flies. A species of Systropus has been recorded as destroying the larva of Limacodes. Several of the Bombyliids of the genus just mentioned are remarkable for the great resemblance they display to various Hymenoptera, some of them being very slender flies, like the thin bodied fossorial Hymenoptera. The difference between the pupa and imago in this case is very remarkable (Fig. 233).
Fig. 233—Systropus crudelis. South Africa. A, Pupa; B, imago, appendages of the left side removed. (After Westwood.)
Fam. 21. Acroceridae or Cyrtidae.—Flies of the average size, of peculiar form, the small head consisting almost entirely of the eyes, and bent down under the humped thorax: wings small, halteres entirely concealed by the very large horizontal squamae; antennae very diverse. The peculiar shape of these flies is an exaggeration of that we have already noticed in Bombylius. The mouth in Acroceridae is very variable; there may be a very long, slender proboscis (Acrocera), or the mouth-parts may be so atrophied that it is doubtful whether even an orifice exists (Ogcodes). There are but few species known, and all of them are rare;[[408]] in Britain we have but two (Ogcodes gibbosus, Acrocera globulus). The genus Pterodontia, found in North America and Australia, an inflated bladder-like form with a minute head, is amongst the most extraordinary of all the forms of Diptera. The habits are very peculiar, the larvae, so far as known, all living as parasites within the bodies of spiders or in their egg-bags. It appears, however, that the flies do not oviposit in appropriate places, but place their eggs on stems of plants, and the young larvae have to find their way to the spiders. Brauer has described the larva of the European Astomella lindeni,[[409]] which lives in the body of a spider, Cteniza ariana; it is amphipneustic and maggot-like, the head being extremely small. The larva leaves the body of the spider for pupation; the pupa is much arched, and the head is destitute of the peculiar armature of the Bombyliidae, but has a serrate ridge on the thorax. Emerton found the larvae of an Acrocera in the webs of a common North American spider, Amaurobius sylvestris, they having eaten, it was supposed, the makers of the cobwebs.
Fig. 234—Megalybus gracilis. × 4. (Acroceridae.) Chili. (After Westwood.)