500 PHYSIOLOGY. 



themselves it is but with the greatest trouble and cleanliness that they 

 can be removed. All day long they repose tranquilly,, and it is only at 

 night that they attack mankind. Towards morning they retire into 

 their retreats, and do not, like the flea, accompany man also by day, 

 secreting themselves between the clothes and the body. They are 

 sensible of all kinds of odours, for example, citric acid, the sweat of 

 horses, assafcetida, sulphur, &c. will drive them away for a time. 



Besides these constant parasites there are a multitude of other insects 

 which as blood suckers annoy man by their punctures : such are the 

 gnats (Cnliccs), the genera Ceratopogon and Simulia (to which, 

 according to all probability, the mosquitos belong, if this name be not 

 applied without distinction to all kinds of small puncturing Diptera, 

 and which thence comprises in different countries very different insects), 

 the Tabani (especially Ckrysops and Hamatopota), and the Stomoxys. 

 It is usually said of the true gnats that the females only sting, but this 

 is incorrect ; it is true that the males are observed less frequently, as 

 they die immediately after pairing, yet do they sting as well as the 

 females, as I have myself observed. 



Many larvae are also classed among the true parasites of man, as they 

 have been found in isolated instances in his evacuations. Kirby and 

 Spence relate several instances in their classical work in which larvae 

 were either cast up or down. Thus the larvae of Tenebrio molitor, of 

 Dermestes (lardarius ?), and of butterflies; and even perfect beetles, 

 for instance, Meloe, have been rejected. According to Azara there is 

 a brown moth in South America which glues its eggs to the skin of 

 sleeping persons ; the young larvee bore into the skin, and here live for 

 a time, until they betray themselves by the pain they occasion, and 

 when they are pressed out. If this be true it can be referred only to 

 the genus (Estrus. Indeed it is said that there is a species of this 

 genus (CEstrus hominis*) the larva of which resides beneath the skin 

 of man, as is confirmed by Humboldt's more recent enquiries f. That 

 maggots of the flies are sometimes evacuated is a frequently occurring 

 fact known to all physicians, and indeed one instance came under my 

 own observation during the short period I practised. The maggots 

 were of the size of the half grown maggots of the flesh fly, and cor- 

 responded with them ; but they were dead when I saw them, other- 



* Gmelin Systema Naturae, vol. i. p. 5. page 2811, No. 10. 

 t Essai sur la Geographic cles Plantes, p. 136. 



