More Minor Horrors

PART II

In Russia the small Asiatic cockroach (P. orientalis) has everywhere driven before it its greater congener (P. germanica). (DARWIN, Origin of Species.)

Cockroaches do a very considerable amount of damage by consuming food-supplies. But they do not stop at food-supplies: woollen clothing, newspapers—not a really great loss—blacking, ink, leather, and even emery-paper, are all to their taste, and, being of an economical frame of mind, they devour their own cast skins and the dead bodies of their relatives. The late Professor Moseley recorded how on one occasion, when on the circumnavigating tour of H.M.S. Challenger, a number of cockroaches took up their abode in his cabin and devoured parts of his boots, ‘nibbling off all the margins of leather projecting beyond the seams on the upper leathers.’ He further records:—

One huge winged cockroach baffled me in my attempts to get rid of him for a long time. I could not discover his retreat. At night he came out and rested on my book-shelf at the foot of my bed, swaying his antennae to and fro, and watching me closely. If I reached out my hand from bed to get a stick, or raised my book to throw it at him, he dropped at once on the deck, and was forthwith out of harm’s way. He bothered me much, because, when my light was out, he had a familiar habit of coming to sip the moisture from my face and lips, which was decidedly unpleasant, and awoke me often from a doze. I believe it was with this object that he watched me before I went to sleep. I often had a shot at him with a book or other missile as he sat on the book-shelf, but he always dodged and escaped. His quickness and agility astonished me. At last I triumphed by adopting the advice of Captain Maclear and shooting him with a pellet of paper from my air-gun, a mode of attack for which he was evidently unprepared.

It is on record that cargoes of cheeses have been destroyed by cockroaches on ships. Not only did they devour great quantities of each cheese, but they defiled every one of them with their very tenacious fluid which has, as we have noted above, a most disgusting smell. This the cockroaches poured out from their stink-glands, making the cheeses of no commercial value.

When a cockroach casts its skin a median longitudinal slit appears on the back of the thorax, and through this slit the insect slowly emerges. With much labour and difficulty it squeezes its body through and pulls one limb after another from its old integument, until at last even the long whip-like antennae are completely withdrawn. Certain portions of its inner anatomy—such as the lining of parts of the breathing-tubes, or tracheae—are also withdrawn. Should the discarded skin not be eaten by the emergent insect, it remains on the floor, and might easily be mistaken for a sedentary cockroach but for the fact that live cockroaches never are sedentary.

The incomplete metamorphosis, the generalised character of the nervures of the hind wings, the complete separation of the three thoracic segments (or rather their want of that fusion so conspicuous in the higher insects—the flies and the bees) and the undifferentiated condition of the mouth parts—all point to the insect being of a primitive type. But there is no doubt that, whether a primitive insect or not, the cockroach is a very successful one; it is an arriviste—as ‘our lively friend, the Gaul,’ to quote Mr. Micawber, would say—probably owing to its attaching itself in all cases, and with unvaried devotion to the habitation of men. Not popular with humanity, it nevertheless ceaselessly extends its domain by slowly yet surely entering into new and hitherto unconquered human habitations. In spite of insect-traps and vermin-killers, it is extremely difficult to eradicate from a house when once it is well established. It has, in fact, gradually dislodged in most places in Great Britain and Ireland the old domestic house-cricket. For in spite of its irritating, and to some people quite maddening, ticking, the ‘cricket-on-the-hearth’ has somehow established itself as a household pet, and one that has won not only our respect but our affection. So curious is our psychology.

The cockroach has many enemies, and the genus Sphex (or Chlorion) may be seen hunting about here and there, up and down the road-side and gardens, searching for its favourite prey. It spies out a cockroach, which appears to know intuitively that there is danger at hand, for it shows symptoms of great fright, and seems so confused that it cannot run away. The Chlorion pounces upon the insect, clasps it with its mandibles between the head and the corselet, and stabs it in the body with the sting. Then it flies off for a little distance, and awaits the effects of the poison thus introduced; and when the convulsions of the victim have ceased, the clever little insect seizes its stupefied prey, and drags the heavy burden with great efforts to its nest. Usually the opening of the cavity is so narrow that the cockroach cannot be got in, for its legs and wings stick out and prevent its introduction. But the Chlorion sets to work and cuts off the legs and the wings, and having thus lessened the difficulty, it strives hard to push the body into the hole; but as this plan usually fails, the hymenopteron enters first of all, seizes the cockroach with its mandibles, and drags it in with all its force. As the integuments of the Blatta are more or less soft and flexible, the great insect is at last forced into the gallery, where it never could have been expected to have entered. Such proceedings on the part of the Chlorion almost verge upon the domain of reason; and it is difficult to explain them by the notion of that very indefinite quality called instinct, for the manœuvres vary according to the circumstances, and there appears to be an intelligent method of overcoming every difficulty.

Apart from animals which eat it, there are a number of parasites which infest it, beginning with the parasitic beetle Symbius blattarum, whose wingless females attach themselves to the bodies of the cockroaches and feed upon their tissues. Then occasionally a round-worm, Filaria rhytipleurites, whose sexual stage is passed in the rat, is found in its larval stage in the fat bodies of the cockroach.

Two years ago Dr. C. Conyers Morrell undertook some investigations and observations as to what part, if any, cockroaches played in the dissemination of pathogenic microbes, his object being, as he says, ‘first to ascertain what bacilli belonging to the colon group are likely to be conveyed to food and milk by this insect, and secondly to find whether known bacteria and moulds can be transmitted by the faeces.’ Dr. Conyers Morrell’s experiments were conducted on one of the Union Castle liners sailing to South Africa, and the insects which were investigated were collected only from the larder or passages adjacent to the kitchens; in no case were they taken from lavatories or from staterooms. The general condition of the ship, which was almost new, was one of exceptional cleanliness, and thus afforded good conditions for the experiments. Dr. Morrell was of opinion that there was little danger except by contamination from the faeces of the infected insect.

The Transformation of Insects, by P. M. Duncan. London: Cassell, Petter, Galpin and Co., 1882.

One of his first experiments was to prove that should cockroaches fall into the dough which was being baked for bread the heat of the baking entirely destroyed the bacilli that were in the alimentary canal of the insect. With regard to infection with the colon bacillus, he kept an infected insect under the best antiseptic conditions he could compass until it had passed some undigested food. Of this undigested food an emulsion was prepared, and cultures were made from it on bile-salt medium and in litmus-milk. Afterwards special cultures were made in gelatine and peptone solutions. Incubation was conducted in all cases at 37° C., and cultures were made from seventeen specimens. Five of these produced colonies of bacilli on the bile-salt medium, with sub-culture results as follows: Four produced acidity and clotting of milk, acid, and gas in glucose, lactose, and saccharose, and production of indol. But the bacilli did not liquefy gelatine, and were Gram-negative. One specimen produced gas in glucose and lactose, and liquefied gelatine and coagulated milk. The former in its reaction corresponded to the Bacillus lactis aërogenes, the latter to Bacillus cloacae. In five cases greenish moulds of the Aspergillus variety were found after inoculating litmus-milk.

Cockroaches will devour human sputum with avidity, and are frequently to be found in spittoons (or, as the more delicately minded American calls them, ‘cuspidors’), and it is interesting to know that after feeding the insects on infected sputum from a tuberculous patient, the tubercle bacilli are found in the faeces within twenty-four hours; two specimens which had been fed on staphylococci showed these pathogenic organisms in their faeces and in the cultures on agar-agar, which were obtained from their dejecta.

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