ARTHROPODA 271 



named as an example, are larger animals with tracheae and 

 often with eyes. Their larvae generally live on and among 

 plants, but presently seize hold of passing animals, generally 

 warm-blooded vertebrates, and proceed to suck their blood. 

 Sometimes they pass their stages on this host, sometimes 

 they fall off and seek another for adult life. The host's 

 skin is pierced by a rostrum formed by the pedipalps, which 

 contains the chelicerae and a mid-ventral piece, the hypos- 

 tome, which is barbed, so that the parasite cannot be pulled 

 off. The female swells greatly by gorging blood. The male, 

 which is smaller, swells less. Fertilisation takes place on 

 the host, and the female then falls off and lays her eggs. 

 Ticks spread the minute parasites which cause various 

 serious diseases, such as red-water fever in cattle, heart-water 

 in sheep, and tick fever in man. 



