PAHASITES FREE DURING THEIR WHOLE LIFE. 117 



our daily task only by the force of habit ; we can neither 

 speak nor think. When the musquitoes disappear, the 

 * black-flies ' come : the musquito pumps up a drop of 

 blood and flies away ; the black-fly bites and makes a 

 wound which continues to bleed." 



De Saussure has alluded to cm-ious relations wdiich 

 exist in Mexico between a bird, a beast, and an insect. 

 " Bulls bury themselves in the mud," says this learned 

 traveller, "in order to avoid the attacks of gnats, leaving 

 in the air only the tip of their nostrils, on w^hich a 

 beautiful bu'd, the Commander, posts himself, in this 

 position the Commander watches for the Maringouin 

 which is bold enough to enter the nostrils of the 

 animal." 



Gnats are parasites in the same manner as leeches, 

 since, like them, they suck the blood, and live at the 

 expense of others. There is, however, this difference, 

 that the females only are greedy of blood ; if this fail 

 them, they live, like the males, on the juices of flowers. 

 Another difference is that they are completely harmless 

 till they have wings, and though they Hve long under 

 their first form, in damp earth or in water, the duration 

 of their life as perfect insects is of short duration. 



We need not trouble ourselves about the active larvae 

 which swarm in stagnant water, nor the chrysalids 

 which float immovable in their natural sepulchre. We 

 give on the next page a representation of a larva of the 

 gnat. The females alone pierce the skin by means of an 

 auger with teeth at the end; they suck the blood, and 

 before they fly away, distil a liquid venom into the 

 wound. This bite seems to have an anaesthetic effect, 

 which does not cause it to be felt till some time after. 



