More Minor Horrors

Part II

The female imago hibernates. Finsch made observations and found it hibernating on the frozen Siberian tundras, beneath the moss and snow. Sterling found them in North America when the snow was melting, in great numbers, and he and his party were subsequently terribly bitten. There is no doubt that female imagines live throughout the winter, and they can be found in England, hibernating in cellars, old out-houses, chicken-houses, or disused farm buildings. These hibernating females disappear early in May, presumably having laid their eggs. Dr. Thayer of Baltimore describes these creatures, having found them on the roofs and walls of barns near New Orleans. Whether the male also hibernates is doubtful. Grassi says he never found the male of A. maculipennis in the winter, only fertilised females. But as the warm weather sets in the female generally becomes active and bites, and the native American Indians consider these elderly and famished females give more annoyance than at any other stage in the life-cycle of either sex. In the warmer climate of Southern Italy they not infrequently hibernate in grottos and caves. At times they occur in such numbers that they can be swept up. After depositing their eggs the hibernating females probably die. This usually happens in May.

In the old days we used to collect gnats, keep them in a receptacle unprovided with any food, and when, after a couple of days, they died of starvation we wrote poems or essays on the ‘Transitoriness of Life’ and the ‘Evanescence of Time.’

Nowadays, we feed them. Bananas, sweetened milk, pineapple, or almost any other vegetable juice, is their diet, and in captivity they will live for weeks. At Cambridge in 1900 (July to August), Professor Nuttall was successful in keeping females alive on a diet of bananas and water from two to eight weeks, but it was found essential to keep the atmosphere fairly moist and the food fresh. Grassi found that he could only keep Anopheles alive in his laboratory in Rome for a month.

Both Anopheles and Culex—at any rate, in captivity—lay their eggs early in the morning. Apparently the nature of the food has some effect upon their fertility, certain observers stating that when male and female are fed on vegetable food alone there is no fertilisation and no oviposition. A diet of blood evidently assists the female to lay her eggs, and perhaps to get them fertilised. One of our female Anopheles laid a batch of 146 eggs, and subsequently laid six more. But, as a rule, a fertilised female does not lay a second batch unless she receives a second meal of blood. The eggs are laid two or three days after the meal. There is also some evidence that a meal of blood is necessary if fertilisation is to be effected. As Austen says in The Report of the Sierra Leone Expedition of the Liverpool School of Tropical Medicine:—

Undoubtedly, if mosquitos ever talk, they would talk like Mr. Waterbrook, Mrs. Henry Spiker—Hamlet’s aunt—and the ‘simpering fellow with weak legs’ talked when David Copperfield dined with the first-named at Ely Place, Holborn. The burden of their song was: ‘Give us blood.’

But a word of caution must be given here. Most of these deductions are based upon mosquitos in captivity; whether the same be true of them in natural conditions is not quite certain. If it be so it is difficult to see how these countless millions of gnats and mosquitos which dwell in the barren regions around the polar circle ever keep going.

It very frequently happens in the Animal Kingdom that females are much more numerous, as well as much larger, than the males.[5] As Kipling tells us: ‘The female of the species is more deadly than the male,’ but Professor Nuttall and I did not notice that this was the case with Anopheles.

There is some evidence that the male hatches out earlier than the female, and that in Southern Europe there may be three or four generations in the course of the season: the first beginning in April and the fourth taking place between the middle of September and the middle of October. After that date no larvae were found. About four generations also occurred in the neighbourhood of Cambridge, according to observations of Professor Nuttall.

Kerschbaumer has calculated that if the average number of eggs laid by a female be 150, the number of the descendants by the fourth generation would amount to 31 millions. This readily accounts for the fact that in certain parts of the world they occur in perfectly enormous numbers, and if it be true that blood is essential for fertilisation and oviposition, very few of these potential mothers can breed. In nature they will feed on a great number of vegetable juices—melons, wild cherry-blossom, bananas, oranges, overripe mangoes; they suck the ‘juices’ of allied species of insects just when the imago is issuing from the pupa-case and before their integument is hardened, or they pierce the soft skin of the cicada, and occasionally attack the chrysalids of a butterfly. One of the most curious sources of food are very young trout. The adult insect attacks these petits poissons filiformes, ‘literally sucking out their unsuspective little brains before they could escape.’ Grassi is doubtful whether the adult males feed at all. He states that he never found any food in their stomach, nor has he ever seen a male feed. But Professor Nuttall’s experiments in Cambridge prove that males were seen repeatedly to feed, and to feed hungrily, on cherries, dried fruits, dates, and bananas.[6]

As mentioned before, the proboscis of the male is too weak to pierce the human integument, but Howard notes that it will suck up water, molasses, and beer; and Gray, at Santa Lucia, mentions that in that island Culex had developed a marked fondness for port wine. One particularly favourite food is rose-buds covered with aphides—probably due to the sweetened secretion which these insects exude. The feeding is sometimes very ravenous, so that the insects become distended, the bright colour of blood, or coloured sap, readily shining through the joints of their chitinous armour.

The reaction to heat and cold is that common to many insects. During the winter the imagines become torpid, quiescent, and cease to worry one. With returning warmth they become lively again, and generally wake from their winter sleep in a state of considerable hunger. They are insects which prefer darkness to light, and during the day-time congregate in caverns and grottos, under the shade of trees and bushes, beneath bridges, in barns, and so on. As the sun sinks they emerge from their hiding-places and fly during the night.

Cambon, writing on A. maculipennis found in the Roman Campagna, says that imagines ‘appear a few minutes after sunset and disappear a few minutes before sunrise.’ We were able to confirm this at Cambridge. The insects retired into the shadiest parts of the boxes in which they were living until the time of sunset, when a loud buzzing was heard, and the insects promptly fed on the food which they had neglected during the day. We kept our tame mosquitos in a huge gauze tent, and at night they invariably accumulated on the side which was illuminated by a lamp. Such mosquitos as were kept in a glass lamp-chimney, closed with gauze at each end, invariably flew towards the end which was held towards the light. People who are experienced with mosquitos sometimes keep the room in which they are sleeping dark and place a light in an adjoining room, leaving the door ajar, and thus lure them away. It seems a curious thing that, while these insects are repelled by the diffused light of the sun, they are attracted by the more concentrated light of a lamp or candle, but such is the psychology of Anopheles.

It is not perhaps solely the influence of light; it may be the influence of colour; for light is very rarely entirely colourless. In the many experiments carried on in Cambridge on the natural history of the mosquito, A. maculipennis, not the least interesting were those directed to ascertaining the insect’s preference for colour. It had been noticed by many observers that they frequented dark-coloured areas rather than light: for instance, note how few mosquitos there are on the white collar of the gentleman in the Frontispiece compared with the number on his dark head and coat. Austen had pointed out that in a room with a dark dado it was on the dado that the mosquitos were found rather than on the whitened walls above. Buchanan noted that the men when collecting Anopheles in an Indian hospital found they were to be most easily got by hanging up a dark coat or two upon the walls. A white coat they always avoided. The proverbial yellow dog of the West is much less bitten than the Newfoundland, and persons wearing dark socks and black shoes are more bitten than those who wear light ones. Natives, although they suffer less in health having acquired a certain immunity, are undoubtedly more bitten than the Europeans.

The experiments we carried on at Cambridge were as follows: In the large gauze cubical tent in which the mosquitos were bred and kept, a number of pasteboard boxes without lids, measuring 20 by 16 by 10 cm., were piled up. The boxes were lined with seventeen different coloured cloths, and were placed in rows one above another, and the order was changed each day, so that no question of height from the floor or better illumination entered into the problem. Counts were made of the inhabitants of each box on each of seventeen consecutive days, with the following results:—

It will be noted that about the level of the pearl grey there was a marked drop. Pale green and pale light blue, ochre, white, orange, and yellow—especially the last two colours—seem positively to repel the insect. Our khaki-clothed soldiers have other advantages than invisibility to the foe. This matter is worth pursuing farther, and it might be possible to design mosquito-traps lined with navy-blue; by periodically exposing them to chloroform or benzine, or by sweeping out the contents, considerable numbers of mosquitos might be destroyed. A dark blue, sticky solution might be even more effective. After reading this chapter in the British Medical Journal, Mr. J. Cropper of Chepstow wrote to me as follows:—

The best way of ‘downing’ mosquitos is to prevent the imago hatching, and this, as has been indicated, can be done by killing the larvae and the pupae, which is effected by brushing oil on the water in which they live. The petrol or crude mineral oil should be renewed from time to time as it evaporates. When once the mosquitos are hatched, every effort should be made to keep them outside dwelling-houses by means of wire screens, but if that be impracticable mosquito-nets should be used at nights. Professor Lefroy recommends one with sixteen to eighteen meshes ‘to the inch.’ They may be driven away from a room by burning pyrethum powder in it, or vaporising cresol or carbolic acid, but of course this must only be done when a window is open, through which they can escape. As regards the human body, mosquitos may to some extent be kept away by smearing the skin with the various essential oils—such as eucalyptus oil or lemon-grass oil, &c. Mosquitos not infrequently bite through the socks, but wearing two pairs of socks instead of one pair, or inserting paper under the socks, often prevents their reaching the skin, as the proboscis is not long enough to penetrate two woollen socks, or strong enough to pierce the paper.