But their most remarkable personal parasites consist of some very extraordinary insects, so anomalous in their structure as to have required the construction of an order for their reception,—the Order Strepsiptera, or “twisted-winged,” thus named from the twist taken by their anterior wings or wing-cases. Their natural history is but imperfectly known, and I believe the males have not yet been discovered. Their larva lives within the bee, and feeds on its viscera by absorption, being attached within by a sort of umbilical cord. It presently consumes the viscera, and renders the bee abortive, by destroying its ovaries, for it is usually upon female bees that it is found. When full fed it forms a case within which it changes into the pupa and imago, the head of which case protrudes between the scales of one of the dorsal segments of the abdomen. How it becomes deposited within the bee or the bee’s larva remains a mystery, although many hypotheses have been hazarded to account for it, but all are unsatisfactory. The Order consists of three genera (Stylops, Elenchus, and Halictophagus) found in England, and other parts of Europe; indeed, the genus Elenchus has been also discovered in the Mauritius. The Continent possesses the genus Xenos, of the same order, and parasitical upon a wasp, neither of which occur with us.
Mr. Kirby, in studying the bees for his invaluable ‘Monographia Apum Angliæ,’ first came across this extraordinary creature. His description of his discovery is highly interesting. He says, at page 111 of volume ii. of the above work, that having observed a protuberance upon the body of the bee, he was anxious to ascertain whether it might be an Acarus, and goes on: “What was my astonishment when, upon attempting to disengage it with a pin, I drew forth from the body of the bee, a white fleshy larva, a quarter of an inch long, the head of which I had mistaken for an Acarus. How this animal receives its nutriment seems a mystery. Upon examining the head under a strong magnifier, I could not discover any mouth or proboscis with which it might perforate the corneous covering of the abdomen, and so support itself by suction; on the under side of the head, at its junction with the body there was a concavity, but I could observe nothing in this but a uniform unbroken surface. As the body of the animal is inserted in the body of the bee, does that part receive its nutriment from it by absorption? After I had examined one specimen, I attempted to extract a second, and the reader may imagine how greatly my astonishment was increased, when, after I had drawn it out but a little way, I saw its skin burst, and a head as black as ink, with large staring eyes, and antennæ consisting of two branches, break forth, and move itself briskly from side to side. It looked like a little imp of darkness just emerging from the infernal regions. I was impatient to become better acquainted with so singular a creature. When it was completely disengaged, and I had secured it from making its escape, I set myself to examine it as carefully as possible; and I found, after a careful inquiry, that I had not only got a nondescript, but also an insect of a new genus whose very class seemed dubious.”
As everything connected with so strange a creature is very attractive, I will cite what other observers also have seen. Mr. Dale, from whom Curtis received Elenchus to figure in his ‘British Entomology,’ vol. v. pl. 226, says: “These parasites look milk-white on the wing, with a jet-black body, and are totally unlike anything else. It flew with an undulating or vacillating motion amongst the young shoots of a quickset hedge, and I could not catch it until it settled upon one, when it ran up and down, its wings in motion, and making a considerable buzz or hum, as loud as a Sesia; it twisted about its rather long tail, and turned it up like a Staphylinus. I put it under a glass and placed it in the sun; it became quite furious in its confinement, and never ceased running about for two hours. The elytra or processes were kept in quick vibration, as well as the wings; it buzzed against the sides of the glass with its head touching it, and tumbling about on its back. By putting two bees (Andrena labialis) under a glass in the sun, two Stylops were produced: the bees seemed uneasy, and went up towards them, but evidently with caution, as if to fight; and moving their antennæ towards them, retreated. I once thought the bee attempted to seize it; but the oddest thing was to see the Stylops get on the body of the bee and ride about, the latter using every effort to throw his rider.
“As the Stylops emerges from the body of the bee, the latter seems to suffer from much irritating excitement.”
Mr. Thwaites writes to me, on the 12th May, thus: “I had the good fortune to capture a Stylops flying, and on the Tuesday following saw at least twenty flying about in the garden, but so high from the ground that I could capture only about half-a-dozen; since that time they have become gradually more scarce.
“The little animals are exceedingly graceful in their flight, taking long sweeps as if carried along by a gentle breeze, and occasionally hovering at a few inches distance from the ground. Their expanse of wing and mode of flight give them a very different appearance to any other insect on the wing. When captured they are exceedingly active, running up and down the sides of the bottle in which they are confined, moving their wings and antennæ very rapidly. Their term of life seems to be very short, none of those I have captured living beyond five hours, and one I extracted from a bee in the afternoon was dead the next morning.
“All the bees stylopized, both male and female, I have taken, have manifested it by having underneath the fourth (invariably) upper segment of the abdomen a protuberance which is scale-like when the Stylops is in the larva state; but which is much larger and more rounded when the Stylops is ready to emerge. A bee gives nourishment generally to but one Stylops; but I have occasionally found two, and once three larvæ in one bee.”
The structure of these insects is very remarkable: the typical genus Stylops is named from its compound eyes, which consist of a very few (about fifteen) hexagonal facets, seated upon a sort of footstalk. The mandibles are lancet-shaped and very acute, and the head, by reason of the protuberant eyes, has very much the shape of a dumb-bell. The antennæ are branched, but in Halictophagus, they are flabellate. The thorax is greatly developed; the superior wing is like a rudimentary wing-case, and is twisted, the inferior wings are very large, and fold along the abdomen in repose like a fan; the legs are slender, and the tarsi with four joints in Stylops, with three in Halictophagus, and with two in Elenchus; the abdomen is long, very flexible, and consists of eight segments. The insects themselves do not exceed a quarter of an inch in length in the largest, but they are generally very much smaller. The perfect insect is very short-lived, not surviving many hours, as just stated. They are usually found in the months of May and June, and they have been discovered to infest several species of Andrena and Halictus, for instance the A. nigro-ænea, upon which Mr. Kirby first found it; A. labialis, which I have frequently caught stylopized; A. rufitarsis, fulvicrus, Mouffetella, tibialis, Collinsonana, varians, picicornis, nana, parvula, xanthura, convexiuscula, Afzeliella, Gwynana, etc., and upon Halictus æratus, etc.
The other mode of parasitism destructive to the bees is where the parasite deposits its own egg upon the provender stored by the bee for the sustenance of its own young. The young of the parasite, either by being more speedily hatched or more rapacious than the larva of the sitos, starves the latter by consuming its food. This kind of parasites consists of several Diptera, but they are mostly bees which form a distinctive subsection of the family of true bees (Apidæ), the subsection being called the Nudipedes or naked-legged, from their not having the necessary apparatus of hair upon the posterior thighs or shanks, for the conveyance of pollen wherewith to store their nests. Thus nature, having rendered them unable to perform this duty to their offspring, has imposed upon them the necessity of resorting to strangers to support them, and they are not led to it by idleness or indifference. These insects consist, with us, of six genera, the species of which are individually attached to some particular bee, who thus nurtures their young. They are, as a rule, gayer insects than those which they infest, and the genus most abundant in species is Nomada, which attaches itself chiefly to Andrena, although some of its species, especially the smaller ones, infest the species of Halictus, and one frequents Eucera. Melecta appears confined to Anthophora; Epeolus to Colletes; Stelis perhaps to Osmia, judging from the great similarity of habit; and Cœlioxys to the constructive Megachile. None of these parasites resemble their sitos, but Nomada is exceedingly different, being in its gay array more like a wasp than a bee. The only close approach in the appearance of a parasite to the insect upon which it is parasitical is in the resemblance between Apathus and Bombus, which are so alike that they were long continued to be united in the same genus, until the peculiar characteristic of the parasitical bees was detected, when they were readily separated. Although, cuckoo-bees as they are familiarly called, they could not be associated with the Nudipedes, because their posterior legs, though not pollen-conveying organs, are hairy; but the Cenobites, to which section they belong, have a peculiar and distinguishing structure of that limb. They are further separated from the Nudipedes by several frequenting the same nest, thus habitually associating with their sitos. Some of the Chrysididæ are likewise, as I shall have occasion to notice in the description of the habits of the genera, similarly parasitical upon some of the species of the family of bees. The genus Mutilla is also probably entirely parasitical upon bees, for Mutilla Europæa is a parasite upon Bombus lapidarius, from whose nests it has been dug in winter, by my friend the late Mr. Pickering, whose activity and accurate observation once promised to be very beneficial to the science, but he, like many others of my entomological friends, is now no more!