I have observed more than once, that the flesh-fly and some others of the same tribe are subject in particular seasons to a kind of convulsions. When thus attacked, they kick and struggle, and seem unable to fly. Sometimes they lie upon their backs without motion, but if a finger be placed near them their convulsive motions are renewed. When thrown into the air, instead of flying, they fall to the ground. Had this distemper occurred earlier or later in the year I should have attributed it to the benumbing effects of cold; but as my observations were made one year (1816) in May, and in another (1811) in the latter end of June, this could scarcely be the case. In the year last mentioned I observed that many flies died under its influence. In wet seasons this tribe is subject to another disease, which proves fatal to many of them, and indeed to other Diptera. A white crust appears to be formed upon the abdomen both above and below, of a granular appearance, much resembling fine moist sugar. On the back of that part this crust does not cover the margins of the segments, which gives it the appearance of white bands; so that deceived by it, I have often at first flattered myself that I had met with some new species. The under-side of the abdomen is wholly covered by it, divided in the middle into two longitudinal masses, the anal segment being bare. De Geer has noticed this or a similar disease, which, when flies are attacked by it, causes the abdomen to swell so as even to burst, and the segments become dislocated. Upon opening the abdomen it is found filled with a white unctuous substance, which often accumulates (as above described) on its external surface[933]. Dr. Host says that in this disease when the animal is dead, the wings, which were before incumbent, become extended, and its almost invisible pubescence grows into long hairs[934]. De Geer seems to think that these flies are thus affected in consequence of having eaten some poisonous food[935]; but I rather suspect, as I have observed it become prevalent chiefly in wet seasons, that it arises from a superabundance of the nutritive fluid, or of the fat, so that it seems to be a kind of plethora. I once observed a fly fixed to a pane of glass, round which was a semicircle of what appeared to be merely vapour, whose radius was nearly three-fourths of an inch. Taking it for an aqueous fluid that had transpired from the dead animal, I paid no further attention to it at that time: but observing from day to day that the moisture did not evaporate, after two or three months had elapsed, I had the curiosity to examine it more closely, and, upon scraping some of it off with a penknife, I found it was a white substance of a fatty nature. In this case, then, the fat must have exploded on all sides with considerable violence from half the body or the abdomen. Probably this was a more intense degree of plethora. When I examined this appearance the fly had fallen off, and I could not find it.
Mr. Sheppard once brought me a panicle of grass, the glumes of which were rough with hairs, or small bristles, to which several specimens of a fly related to Xylota pipiens adhered by their proboscis. At first I thought that having been entrapped by the bristles, and unable to extricate themselves, they had perished from want of food; but since when touched they readily dropped from the glumes, some other cause, perhaps disease, probably occasioned this singular suspension of themselves.
The maladies to which bees and silkworms are subject are more interesting to us than those of flies, on account of their utility as cultivated insects. One of the worst distempers which attacks the first of these animals is a kind of looseness or dysentery: this happens early in the year, when they are fed with too much honey without any portion of bee-bread[936], and sometimes destroys whole hives. Their excrements, instead of a yellowish red, then become black, and the odour they emit is insupportable; the bees no longer observe their usual neatness, inducing them to leave the hive when they void their excrements, but they defile it, their cells, and each other. Several remedies have been prescribed for this disease. To prevent it, a syrup made by an equal mixture of good wine and honey is recommended; and as a cure, to place in the hive combs containing cells filled with bee-bread[937]. But one of the worst maladies to which these useful animals are subject, is that called by Schirach Faux Couvain. It originates with the larvæ; and is caused either by their being fed with unwholesome food, or when the queen, as sometimes happens, lays her eggs so that the head of the grub is not in a proper position for emerging from the cell when the period for its disclosure is arrived:—the consequence is, that in both cases it dies and becomes putrid, which sometimes produces a real pestilence in a hive. The remedy for this evil is to cut away the infected combs, and to make the bees undergo a fast of two days[938]. The hive should be cleaned and fumigated, by burning under it aromatic plants.
The cultivators of the silkworm in France have given names to several diseases to which that animal is subject. One is called La Rouge, and is supposed to be occasioned either by too great heat, or by too sudden a transition from cold to heat. It takes place when the caterpillar is first hatched; which lives perhaps, but in a very sickly state, till it should spin its cocoon and assume the pupa, when it expires. Another degree of the same disease is called Les Harpions or Passis. A second distemper of this animal is Des Vaches, Le Gras or La Saune: this is a mortal disease, supposed to be of a putrid nature, and produced by mephitic air; it shows itself after the second moult, but rarely after the subsequent ones. When a caterpillar is first attacked, changing the air may prove a remedy; but when the disease has made progress, it is best to burn or bury them, since if the poultry pick them up they might be poisoned by them. A third disease of silkworms is called Les Morts Blancs, or Tripes, which is also occasioned by impure air, when the leaves the animal feeds upon are heaped so as to produce fermentation. The caterpillars attacked by it die suddenly, and preserve after their death the semblance of life and health. Too great heat, whether artificial or natural, occasions La Touffe, a fourth, which, when the heat continues long, destroys all those that are arrived at their last stage of existence in their larva state. Black points scattered over different parts of the body, or livid and blackish spots in the vicinity of the spiracles, followed by a yellowish or reddish tint, are symptoms of a fifth malady, called La Muscardine. After this the animal soon dies, and becomes mouldy, but does not stink. This disease is not contagious, and is thought to be caused by a moist heat, attended by pernicious exhalations. La Luzette, Luisette, or Clairène, is another malady, which shows itself most commonly after the fourth moult. It seems to arise from some original defect in the egg. The caterpillars attacked by it may be known by their clear red and afterwards dirty white colour; their body becomes transparent, and the matter of silk exudes in drops from their spinnerets; consequently, though as voracious as the rest, they are never able to construct a cocoon, and should be destroyed. Les Dragées is the name given to cocoons which include a larva that never becomes a pupa. The cause of this disorder has not been ascertained, and whole broods are sometimes subject to it, which, as in the last, seems to imply some defect in the eggs. But as the caterpillar spins its cocoon, and the silk is as good as usual, it is a malady of no great importance. Lastly, sometimes the mulberry leaves have a gummy rather acrid secretion, which purges the silkworms; their excrement is no longer solid; they become weak and languid; and if the secretion is abundant, their transpiration is impeded, and at the time of moulting they are become so feeble as to be unable to cast their skin[939].
In the case of many caterpillars of Lepidoptera that died, Bonnet found by dissection that the disease was remotely occasioned by a diarrhea, which taking place immediately before they became pupæ, prevented the inner membrane of their intestines from being rejected, as it would have been if no extraordinary cause had prevented it, attached to the hard excrement. He found this membrane converted into a jelly occupying great part of the stomach, which he conjectured was the proximate cause of their death[940].
To conclude this head—spiders are reputed to be subject to the stone: I do not say Calculus in Vesica; but we are informed by Lesser that Dr. John Franck having shut up fourteen spiders in a glass with some valerian root, one of them voided an ash-coloured calculus with small black dots[941].
II. I now come to that class of diseases which appears to prevail almost universally amongst insects—I mean those resulting from the attack of parasitic enemies. Thus millions and millions annually perish before they have arrived at their perfect state. Diseases of this kind proceed either from vegetable or animal parasites. I shall begin with the first, which will not occupy us long.
i. As insects pass often no small portion of their life in a state of torpidity, in which they remain chiefly without motion, it will not seem wonderful, should any partial moisture accidentally accumulate upon them, that it affords a seed plot for certain minute fungi to come up and grow in. Persoon observes with regard to his genus Isaria, that one species grows upon the larvæ of insects (I. truncata), and another upon pupæ (I. crassa[942]):—as he does not say upon dead larvæ and pupæ, as upon a former occasion[943], perhaps in these cases these plants may constitute an insect disease; but I lay no stress upon it, and only mention the circumstance here as connected with the history of these animals. Mr. Dickson has described a Sphæria under the name of entomorhiza that grows upon dead larvæ; it has a slender long stipes and spherical granulated head: on the pupa of a species of Cicada in my cabinet, another kind of Sphæria, with a twisted thickish stipes and oblong head, springs up in the space between the eyes. I observed something similar but longer, in the grub of some large beetle in M. Du Fresne's museum at Paris; and I have a memorandum of having noticed something of the kind on the rostrum of a Calandra. Bees and humble-bees have been sometimes thought to have some species of mucor or other Fungilli occasionally growing upon them; but Mr. Brown is of opinion that stamina which they have filched from flowers have been mistaken for these Fungilli, since he has detected those of Orchideæ in some of this tribe, and upon a beetle shown to him by Mr. MacLeay, one which he knew to be the stamen of an Aristolochia. I once observed a bunch of what I mistook for a singular mucor that adorned the vertex of a humble-bee, between the antennæ, which doubtless were of the same description; and I even saw one upon its wing. Upon a former occasion I mentioned a parallel circumstance with respect to a species of Xylocopa[944].
ii. The animal parasites that infest insects are either themselves insects; or worms.
1. Their insect infesters, as far as we know at present, are confined to the Orders Strepsiptera, Hymenoptera, Diptera, and Aptera: they attack them sometimes in their egg state, most frequently when they are larvæ, occasionally when pupæ, and very rarely in their perfect state. Upon many of these I have formerly enlarged[945], and I shall now add such further circumstances as I then omitted. The Strepsiptera Order, as at present known, consists only of two genera, Stylops and Xenos; the first being appropriated to the imago of Andrena, a kind of bee, and the latter to that of the wasps. Their eggs appear to be deposited in the abdomen of these insects in which they feed, till having attained their full growth they perforate the membrane that connects its segments; and at the proper time their pupa-case bursts, they emerge, and take their flight. Sometimes four or five infest a single bee. Whether the latter dies upon their quitting it I have not been able to ascertain, but from their flying, when the little parasite is very near leaving them, with their usual activity, it should seem that this disease is not mortal; but it probably prevents their breeding: I do not recollect observing the exuviæ of one in a male bee[946].