These are best divided into the Winged and Wingless forms. Of the winged pests Mosquitoes are undoubtedly the most important in the tropics, as they are known to transmit such diseases as malaria, yellow fever, dengue fever and filariasis. The anopheline mosquitoes are those responsible for transmitting malaria, and they can easily be recognized by the facts that their wings are usually spotted and that they appear as it were to stand on their heads when resting on any flat surface. The culicine mosquitoes, which include Stegomyia fasciata, the carrier of yellow fever, do not as a rule have spotted wings and they rest parallel to the surface on which they alight. There are also marked differences in the water stages of these insects which cannot, however, be here considered.
The best way of protecting oneself against mosquitoes is the proper use of an effective mosquito net, the mesh of which should contain at least sixteen holes to the linear inch. Mosquito boots or buskins are also useful, and in certain places hoods for the face and neck, such as the “Mosquinette” variety, will be found of value.
Repellent substances applied to the skin may also be employed. Cassia oil, with brown oil of camphor in vaseline, is useful. Vermijelli, containing a little citronella oil, is fairly effective but is not so pleasant to use. The irritating effect of mosquito bites may be diminished by the application of tincture of iodine. The same measures are effective in the case of some of the biting midges.
The insects generally known as Sand-flies are really owl midges or moth flies, tiny and hairy insects which have been proved to transmit sand-fly fever (see [page 221]). These flies breed in damp places where there is rotting vegetation, dark and damp cellars, cracks and fissures in the soil, tunnels, etc., and they are often very troublesome. The repellents mentioned above may be used to drive them away, as may eucalyptus oil, camphor and tobacco smoke. The ordinary mosquito net is useless against their attacks, and one containing twenty-two holes to the linear inch is required.
Buffalo gnats, which are often called sand-flies, are insects which breed in running water and amongst cataracts and rapids. They are formidable biters although they are not known to cause any disease. The same protective measures are indicated in their case as in that of owl midges.
House Flies.—There are several species of these. They are definitely known to distribute certain harmful bacteria and protozoa and to contaminate food by carrying the organisms on their bodies, wings and legs, or depositing them by regurgitation or in their droppings. Hence it is very important to protect all food and drink from flies and to destroy these insects wherever possible. The subject of their destruction is too large a question to be dealt with here, so all that need be said is that the traveller should provide himself with wire mesh fly covers to screen food and also with pieces of mesh or calico weighted with beads, which can be utilized for covering vessels containing milk or other liquids. It is worth noting that ordinary fish netting hung over a tent door or used to screen the windows or doors of a house will effectively exclude flies, despite its large mesh.
Tsetse Flies.—These are considered under Sleeping Sickness (see [page 234]).
The Congo Floor-maggot Fly.—It is the maggot or larva of this fly which is important, as in many parts of Africa it infests the floors of native huts and it is a blood-sucker feeding at night. So far as is known it does not convey any disease but it is an unpleasant visitor, and if its numbers are great they may extract comparatively large quantities of blood from their unconscious victims.
Prophylaxis.—General cleanliness and enforcement of sanitary measures, the use of high beds, the scrutiny of sleeping mats and blankets in which eggs or larvæ may be concealed. Infected huts can be rendered habitable by firing the ground or by removing the surface soil, which can then be disinfected.
Wingless Pests.—Lice.—These are known to transmit typhus fever. Further, they often cause great cutaneous irritation and are loathsome companions. It is no easy matter to get rid of lice when infection is on a large scale, but very often this is not the case and it is possible to detect their presence only by careful examination. There is no real difference between head lice and body lice; they are simply varieties of the same insect. The presence of head lice may be recognized by the discovery of their eggs, which are known as nits and are minute, yellowish-white, goblet-shaped bodies about the size of a full-stop on this page. Examination for head lice is much facilitated by the use of a fine tooth comb. Persons suspected of harbouring body lice should be examined first for the actual bites of the insect on the skin. Their clothes, and especially their under-garments, should then be carefully examined, attention being more particularly directed to the seams and folds where the eggs, if present, are most likely to be found.