The =Mosquito= family (_Culicidæ_) has been found to be an important if not the sole means of spreading certain serious diseases to man. To this family belong all true gnats or mosquitoes; but the only two genera which have been proved to be able to cause disease are _Culex_ and _Anopheles_. The culex may usually be distinguished by the fact that when alive and at rest its head is below the level of the thorax and abdomen, thus giving the insect a hump-backed appearance, while the body of the anopheles under the same circumstances is all in a line.[10] The =anopheles= is more slender and its head smaller than that of the culex. The anopheles usually confines its blood-sucking operations to the evening and night. During the day it remains in dark corners. It lays its eggs usually in a natural pool or pond on the ground, on the surface of the water. In about two days a minute larva is hatched out. This grows rapidly, assumes the pupa form, from which the perfect insect emerges. The female insect alone is blood-sucking. In about 20 days after birth, it lays from 150 to 200 eggs. Its relation to malaria may be gathered from the following historical sketch. In 1880 Laveran found in the red blood corpuscles of malarious patients minute bodies which he regarded as not bacteria, but a very low form of animal life, possessing amœboid movements. These grew at the expense of the blood corpuscles, deposited a dark pigment, and often assumed the appearance of a “rosace,” a rounded body with little spherules at its circumference. Golgi in 1889 observed differences between the rosaces of tertian and quartan fever, and found that the periods of occurrences of the fever corresponded with the times of maturation of the rosaces. It was concluded therefore, that the rosaces caused the fever by shedding their sporules into the blood. These sporules when thus shed were found to attach themselves to, and grow in, other red blood corpuscles. It is now known that there are three species of the parasite, in one of which the parasites are crescentic in shape. The examination of a drop of blood from a patient now enables a doctor to recognise which of these three forms of malaria he is dealing with.