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:—
| Colour of box | Average number of mosquitos in each box. |
|---|---|
| Navy blue | 108 |
| Dark red | 90 |
| Brown (reddish) | 81 |
| Scarlet | 59 |
| Black | 49 |
| Slate grey | 31 |
| Dark green (olive) | 24 |
| Violet | 18 |
| Leaf green | 17 |
| Blue | 14 |
| Pearl grey | 9 |
| Pale green | 4 |
| Light blue (forget-me-not) | 3 |
| Ochre | 2 |
| White | 2 |
| Orange | 1 |
| Yellow | 0 |
| ——— | |
| Total | 512 |
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:—
Seeing your article on Colour Selection by Anopheles reminds me that I found the dark navy-blue lining of my tent this summer (in Palestine) extremely attractive to mosquitos, almost entirely Anopheles; and when the sun got hot I always noticed an increase in their numbers, presumably as they came from the herbage and trees near by. No one ever slept in the tent, and I never found Anopheles bite in the day-time.
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.
CHAPTER VI
THE MOSQUITO (Anopheles maculipennis)
Part III
The tiny-trumpeting gnat can break our dream