We have now done with those parasites that produce in insects the disease I have called Scolechiasis[999]: the rest, which belong to the Aptera Order, will afford us examples both of Phthiriasis and Acariasis[1000].
I begin with the first. Mr. Sheppard once brought me a specimen of a bird-louse (Nirmus) which he took upon a butterfly (Vanessa Io): and should such a capture be more than once repeated, it would afford a certain instance of the first of these diseases amongst insects;—but most probably the specimen in question had dropped from some bird upon the butterfly. The only remaining animal belonging to the apterous hexapods that is parasitic on insects, is by many supposed to be the larva of a giant-beetle (Meloe Proscarabæus). I have before alluded to this animal[1001], and shall now resume the subject. Gœdart, Frisch, and De Geer, observed that it deposited in the earth one or two considerable masses, containing an infinite number of very minute orange-coloured eggs adhering to each other, which in about a month were hatched, and produced a number of small hexapods distinguished by two pairs of anal setæ and a proleg, by means of which they could move readily upon glass, as I have myself seen: these little animals precisely corresponded with one found by the latter author upon Eristalis intricarius; and when that fly was placed amongst them, they immediately attached themselves to it, so as to leave no doubt of their identity[1002]. A congenerous species had been detected upon wild bees, and described by Linné under the name of Pediculus Apis. De Geer is so thoroughly to be depended upon for his veracity and accuracy of observation, that we cannot suppose there is any incorrectness in his statement. If the mass of eggs be, as he represents it, of the size of a hazel-nut, it must have been the product of a very large insect: in confirmation of this opinion it may be further observed, that the larva of the kindred genus Cantharis agrees with it in having anal setæ, though it appears to differ in having only two conspicuous segments in the trunk[1003]. Those which infest wild bees make their first appearance upon acrid plants, which the Meloe likewise feeds upon; from whence with wonderful agility they leap upon the Andrenæ, &c. that visit these flowers. Strong, however, as all these facts appear, still we cannot help exclaiming with the illustrious Swede last named, Who could ever have imagined that the larva of this great beetle would be found upon the body of flies,—and we may add, or bees? Who could ever imagine that it would feed like a bird-louse and resemble it so closely? that in the insertion of its palpi it should exhibit a character exclusively belonging to that tribe[1004]? Another circumstance seems to indicate that these hexapods at the time that they take their station in bees or flies are perfect insects—they do not vary in size, at least not materially. Where, we may also ask, if they are to become large beetles, where do they take their principal growth? It cannot be as parasites on the little bees or flies that they are usually found upon; they must soon desert them, and like their kindred blister-beetles, as is most probable, have recourse to vegetable food. What an anomaly in rerum natura! It is much to be wished that some skilful insect-anatomist would carefully dissect the Meloe; or perhaps by digging round the roots of the ranunculuses and other acrid plants the larva of that beetle might be discovered in a later stage of growth, and so this mystery be cleared up. I should observe here, that Scopoli has described three parasites as Pediculi; viz. P. rostratus, coccineus, and Cerambycinus; the first of which Fabricius has adopted under the name of P. Gryllotalpæ, but which are all evidently hexapod Acarina[1005].
Acariasis seems a disease almost as universal amongst insects as Scolechiasis; with this difference however, that Acari most commonly take their station upon them in their perfect state. You have doubtless often observed the common dung-beetles (Geotrupes) covered on the underside of their body with small mites (Gamasus Coleoptratorum) which look as if they were engaged in suction—they are often so numerous that no part is uncovered; they also attack other beetles[1006], and are sometimes found on humble-bees. They are easily disturbed, run with great swiftness, and may often be seen in hotbeds and fermenting dung prowling in search of the stercorarious beetles. But the most remarkable insect of this kind is the Uropoda vegetans: it derives its nutriment from the insects it assails not by its mouth, but by means of a long anal pedicle by which it is attached to them. De Geer found these in such numbers upon a species of Leptura, that its whole body was almost covered with them; they hung from the legs and antennæ in bunches, and gave the animal a most hideous and disgusting appearance. Under this load of vermin it could scarcely walk or move, and all its efforts to get rid of them were in vain: many were attached to its body and to each other by their anal pedicles, but others had cast them off and were walking about. When put into a glass with earth, they began to abandon their prey, so that in a few days it was quite freed from its plagues. He found that these parasites lived long in alcohol[1007].
If you inquire—How are these mites originally fixed by their pedicles? it seems most probable, that as the Hemerobii, when they lay their eggs, know how to place them upon a kind of footstalk, so the parent Uropoda has the same power; and this pedicle appears to act the part of an umbilical chord, conveying nutriment to the fœtus not from a placenta, but from the body of the insect to which it is attached; till having thus attained a certain maturity of growth and structure, it disengages itself and becomes locomotive. Many eggs of the aquatic Acarina (Hydrachna, &c.) are also furnished with a short pedicle by which they are fixed to Dytisci and other water insects. De Geer found some of this description on the underside of the water-scorpion, so thickly set as to leave no void space: they were oval, of a very bright red, and of different sizes on different individuals; whence it was evident that they grow when thus fixed: when hatched or released—for perhaps they may be regarded as fœtuses in their amnios rather than eggs—they cease to be parasitical. Let us admire on this occasion, (piously observes this great Entomologist,) the different and infinitely varied means by which the Author of Nature has endowed animals, particularly insects, for their propagation and preservation: for it is a most extraordinary sight to see eggs grow, and pump as it were their nutriment from the body of another living animal[1008]. As these mites are fixed to the crust as well as its inosculations, they must have some means of forcing their nutriment through its pores.
Another insect, remarkable for its resemblance in some respects to the scorpion—called in this country the book-crab (Chelifer cancroides), from its being sometimes found in books—occasionally is parasitic upon flies, especially the common blue-bottle-fly (Musca vomitoria). They adhere to it very pertinaciously under the wings; and if you attempt to disturb them, they run backwards, forwards, or sideways, with equal facility.
Spiders also are infested by mites. Mr. Briggs once found a very small Theridion, to the thorax of which were attached four oblong bright scarlet mites, each of which was as large as the thorax itself. He afterwards met with another spider still smaller, attacked by two of these swoln parasites, one of which appeared to him nearly equal to the spider in size. This mite was probably either Leptus Phalangii, or Astoma parasiticum.
2. We now come to a perfectly distinct tribe of insect parasites, which belong to that section or order of intestinal worms which Rudolph has denominated Entozoa nematoidea, and Lamarck Vers rigidules[1009]. To this tribe belong the Gordius of Linné and the Filaria of modern zoologists, which from the experiments and observations of De Geer, Dr. Matthey, &c. appear to have been too hastily separated, being really congenerous, and living indifferently in water and in the intestines of insects and other animals[1010]. To this genus belong the guinea-worm (Gordius medinensis[1011]), the Furia infernalis, and several others that are found in various vertebrate animals. These little worms have been discovered in insects of almost every Order; and their attack generally produces the death of the animal, though they appear not to devour those parts that are essential to life[1012]. I once took a specimen of Pœcilus azureus, and upon immersing it in boiling water I was surprised to see what at first I mistook for an intestine, thrust itself forth; but upon a nearer inspection, to my great surprise I found it was one of these worms, thicker than a horse-hair and of a brown colour. Mr. W. S. MacLeay also once found one in Abax Striola. It still remains in my specimen, making it appear as if it had a long tail. De Geer long ago found these worms in grasshoppers[1012]; but Dr. Matthey has given the fullest account of one which infested Acrida viridissima. A friend of his noticing one of these insects which had not strength enough to leap and could scarcely even walk, being struck with the circumstance, caught the animal, upon which its hind legs were immediately detached from it. His surprise was greatly increased when he saw issue from its body a cylindrical worm about two feet and a half in length. Upon being called, Dr. M. soon recognised it for a Gordius or Filaria; and on his putting it into water, it moved in it with great velocity, twisting its long and slender body in all directions. Upon opening the body of the grasshopper, nothing appeared within it but the intestine shrunk up to a thread. A few days after, another was brought, which appeared in full vigour, but its abdomen was enormously distended, and from it another worm was extracted, which remained without motion rolled in a spiral direction: intending to preserve this in spirits of wine—as it had become flat he first immersed it in water, that it might recover if possible its cylindrical form. Upon immersion a movement took place in the animal, and it gradually recovered its plumpness; but it still remained without motion, as if dead, for nearly five days, when another living specimen being brought and placed with it, as soon as water was poured on them, the seemingly dead one began to show by a slight oscillation in its extremities that life was not extinct in it. Fresh water being poured upon it, at the end of the day it had recovered all its strength and agility. He afterwards often repeated the same experiment with a similar result[1013]. From this account it appears that the Gordius or Filaria has a property resembling that of the Vibrio Tritici, so well described and so beautifully figured by M. Bauer[1014], of apparently dying and being resuscitated by immersion in water. How long it can retain this property remains to be ascertained.
De Geer states that he had seen them of the length of two feet[1015]; but they vary considerably in this respect. In ants, in which Gould detected them, he states their length to be not more than half an inch[1016]. In caterpillars, which they sometimes infest, they are longer; in that of Notodonta Ziczac, De Geer found one three inches and a half long[1017]; and Rösel three, of six inches, in that of Deilephila Euphorbiæ[1018]; and in Phalangium cornutum, according to Latreille, they extend to more than seven inches[1019]. In the larva of a Phryganea L. the author first named found one which was more than a foot long, corresponding exactly with the Gordius aquaticus of Linné; being forked at one extremity, brown above, gray below, and black at each end[1020]. These animals appear to die as soon as they leave the body[1021] they have preyed upon; except this happens in water, when their activity has no repose. In this element they give their bodies every possible inflexion, often tying themselves in knots in various places, interlacing and twisting themselves in a hundred different ways; so that when confined in the body of an insect, from their extreme suppleness and power of contortion they find sufficient space wherein to pack their often enormous length[1022]. Linné makes one of their habitats clay; and Mr. W. S. MacLeay finds them very common at Putney in clay at the bottom of pools.
Dr. Matthey asks—How does the Gordius get into Acrida viridissima[1023]? And De Geer—Why do they die after having quitted a caterpillar? and where do they perpetuate their species[1024]? These questions, without further observations, cannot easily be answered. However, it may be supposed that carnivorous insects, such as Harpali, &c. may swallow them when found apparently dead in clay, where the water has been evaporated, or when they have been ejected by other insects; and they may revive in their bodies, as Dr. Matthey found them to do in water. It is not difficult to conjecture that the larvæ of Phryganeæ may meet with them when young in the water, and sometimes unluckily swallow them with their food. Why they become as dead when they emerge from their prey we cannot at present conjecture; but no doubt to answer some wise purpose;—in rainy seasons they probably revive and get into little hollows full of rain-water. Upon De Geer's last question—How they perpetuate their species—at present I can offer no conjecture.