318 



BIOLOGY: GENERAL AND MEDICAL 



Ixodiodea. The eggs usually hatch upon the 

 ground yielding six-legged "nymphs" or larval 

 forms. With the second moult they acquire a 

 fourth pair of legs and become mature. The 

 young ticks climb the stems of plants and there 

 await the coming of some warm-blooded animal 

 to which they quickly transfer themselves. 

 The proboscides of the little ticks cannot reach 



FIG. 112. Pyroplasma bigeminum. Cause of Texas fever in cattle, in 

 stained blood of steer. X 1000. o, Leucocyte; b, normal erythrocyte; c, 

 erythrocyte containing one pair, d, erythrocyte containing two pairs of pyro- 

 plasmata. 



the blood, but their introduction into the skin 

 sets up an inflammatory reaction with some 

 edema and the lymph nourishes them. The 

 bites sometimes suppurate. When full-grown 

 the proboscis easily penetrates the skin, and the 

 parasites slowly distend ' themselves with blood 

 to a surprising extent. The male tick does 

 not seem to be much of a blood sucker, bites 

 occasionally, and is satisfied with little; the 



