METAZOA ARACHNOZOA. 375 



and the feet are provided with claws and suckers. Tyro- 

 glyphus setiferus (PL 931) has long bristles or setae 

 extending from the body. This figure also exhibits the 

 digestive system, though the anus is not seen, as it is sit- 

 uated ventrally near the posterior end of the body. Four 

 eggs are seen on the ventral side (PL 931). 



Another more specialized form is the itch mite, Sar- 

 coptes scabiei Latr. (PL 932, figs. 1-4). The larva (fig. i) 

 has a nearly circular body in which cephalothorax and 

 abdomen are indistinguishable. It is provided with 

 needle-like mandibles, and, like the larvae of most mites, 

 with three pairs of feet, the two forward pairs having 

 suckers for clinging to the host even at this early age. 



The female (fig. 2, upper side; fig. 3, lower side) is 

 essentially like the larva in form, but has four pairs of 

 feet ; the two forward pairs having large suckers (figs. 

 2, 3) and the two hinder pairs long bristles. The male 

 (fig. 4) is smaller than the female, but like her has four 

 pairs of legs ; each of the third pair ends in a sucker, and 

 each of the fourth pair in a bristle. 



The members of one family of mites, the Ixodidae, are 

 usually called ticks. One of the common examples of 

 this group is the cattle tick, Boophilus bovis Riley (PL 

 933' n s - T -6). The six-footed larva (fig. i, greatly 

 enlarged) looks like a minute seed, having a body in 

 which the cephalothorax and abdomen are united in one 

 mass. These larvae fasten themselves on cattle and 

 develop into the adult (fig. 2, 9 , natural size when gorged 

 with food ; fig. 3, 9 , enlarged ; fig. 4, <J). The mandi- 

 bles (fig. 5) are fitted for piercing the hide, and as their 

 hooks extend backward they enable the tick to hold 

 tightly while sucking the blood of its victim. The feet 

 of the tick are also adapted for clinging to its host, the 

 hind feet (fig. 6) having two claws and a sucking disc as 

 well as a double spur. 



Boophilus illustrates the law of acceleration in develop- 

 ment since it is able to lay eggs any time after it is half 



