[LETTER V.]
INJURIES CAUSED BY INSECTS.
INDIRECT INJURIES.
Having detailed to you the direct injuries which we suffer from insects, I am now to call your attention to their indirect attacks upon us, or the injury which they do our property; and under this view also you will own, with the fullest conviction, that they are not beings that can with prudence or safety be disregarded or despised. Our property, at least that part exposed to the annoyance of these creatures, may be regarded as consisting of animal and vegetable productions, and that in two states; when they are living, namely, and after they are dead. I shall therefore endeavour to give you a sketch of the mischief which they occasion, first to our living animal property, then to our living vegetable property; and lastly to our dead stock, whether animal or vegetable.
Next to our own persons, the animals which we employ in our business or pleasures, or fatten for food, individually considered, are the most valuable part of our possessions—and at certain seasons, hosts of insects of various kinds are incessant in their assaults upon most of them.—To begin with that noble animal the horse.—See him, when turned out to his pasture, unable to touch a morsel of the food he has earned by his labours. He flies to the shade, evidently in great uneasiness, where he stands continually stamping from the pain produced by the insertion of the weapons sheathed in the proboscis of a little fly (Stomoxys calcitrans) before noticed as attacking ourselves[231]. This alights upon him sometimes in one place and sometimes in another, and never lets him rest while the day lasts.—See him again when in harness and travelling. He is bathed in blood flowing from innumerable wounds made by the knives and lancets of various horse-flies (Tabanus, L.), which assail him as he goes, and allow him no respite[232]; and consider that even this is nothing to what he suffers in other climates from the same pest. In North America, vast clouds of different species—so abundant as to obscure every distant object, and so severe in their bite as to merit the appellation of burning flies—cover and torment the horses to such a degree as to excite compassion even in the hearts of the pack-horsemen. Some of them are nearly as big as humble-bees; and, when they pierce the skin and veins of the unhappy beast, make so large an orifice that, besides what they suck, the blood flows down its neck, sides, and shoulders in large drops like tears, till, to use Bartram's expression, "they are all in a gore of blood." Both the dog-tick and the American tick before mentioned, especially the latter, also infest the horse. Kalm affirms, that he has seen the under parts of the belly, and other places of the body, so covered by them, that he could not introduce the point of a knife between them. They were deeply buried in the flesh; and in one instance that he witnessed, the miserable creature was so exhausted by continual suction, that it fell, and afterwards died in great agonies[233].
No quadruped is more infested by the gad-fly, sometimes also improperly called the breese[234], than the horse. In this country no fewer than three species attack it. The most common sort, known by the name of the horse-bee (Gasterophilus Equi), deposits its eggs (which being covered with a slimy substance adhere to the hairs) on such parts of the body as the animal can reach with its tongue; and thus, unconscious of what it is doing, it unwarily introduces into its own citadel the troops of its enemy.—Another species (G. hæmorrhoidalis) is still more troublesome to it, ovipositing upon the lips; and in its endeavours to effect this, from the excessive titillation it occasions, giving the poor beast the most distressing uneasiness. At the sight of this fly horses are always much agitated, tossing their heads about in the air to drive it away; and, if this does not answer, galloping off to a distant part of their pasture, and, as their last resource, taking refuge in the water, where the gad-flies never follow them. We learn from Reaumur, that in France the grooms, when they observe any bots (which is the vulgar name for the larvæ and pupæ of these flies) about the anus of a horse or in its dung, thrust their hand into the passage to search for more; but this seems a useless precaution, which must occasion the animal great pain to answer no good end: for when the bots are passing through the body, having ceased feeding, they can do no further injury. In Sweden, as De Geer informs us, they act much more sensibly: those that have the care of horses are accustomed to clean their mouths and throats with a particular kind of brush, by which method they free them from these disagreeable inmates before they have got into the stomach, or can be at all prejudicial to them[235].
Providence has doubtless created these animals to answer some beneficial purpose; and Mr. Clark's judicious conjectures are an index which points to the very kind of good our cattle may derive from them, as acting the part of perpetual stimuli or blisters: yet when they exceed certain limits, as is often the case with similar animals employed for purposes equally beneficial, they become certainly the causes of disease, and sometimes of death.
How troublesome and teasing is that cloud of flies (Anthomyia meteorica) which you must often have noticed in your summer rides, hovering round the head and neck of your horse, accompanying him as he goes, and causing a perpetual tossing of the former[236]!—And still more annoying in Lapland, as we learn from Linné[237], is the furious assault of the minute horse-gnat, (Culex equinus, L.) which infests these beasts in infinite numbers, running under the mane and amongst the hair, and piercing the skin to suck their blood.—An insect of the same genus is related to attack them in a particular district in India in so tremendous a manner as to cause incurable cancers, which finally destroy them[238].—But of all the insect tormentors of these useful creatures, there is none more trying to them than the forest-fly (Hippobosca equina). Attaching themselves to the parts least covered with hair, particularly under the belly between the hind legs, they irritate the quietest horse, and make him kick so as often to hazard the safety of his rider or driver. This singular animal runs sideways or backwards like a crab; and, being furnished with an unusual number of claws, it adheres so firmly that it is not easy to take it off; and even if you succeed in this, its substance is so hard, that by the utmost pressure of your finger and thumb it is difficult to kill it; and if you let it go with life, it will immediately return to the charge.—Amongst the insect plagues of horses, I should also have enumerated the larva of Lixus paraplecticus, which Linné considers as the cause of the equine disease, called in Sweden, after the Phellandrium aquaticum, "Stâkra," had not the observations of the accurate De Geer rendered it doubtful whether the insect be at all connected with this malady[239].