G. H. F. Nuttall, W. F. Cooper and L. E. Robinson 153 
host to another. There can be no doubt but that ticks will be found, 
upon further investigation, to be associated in the transmission of an 
increasing number of diseases in animals. 
Ticks belong to the class Arachnoidea, in which are included the 
scorpions, spiders and mites, and almost all the members of the class 
possess eight legs when they reach the adult stage. Ticks constitute a 
well-defined super-family, the Ixodoidea, in the order Acarina, and in 
the light of recent knowledge may be divided into two families, the 
Argasidae and the Ixodidae. 
It would be out of place to dwell here upon the particular charac¬ 
ters possessed by the different genera, especially as these will be duly 
considered in a systematic work which is being prepared for publication. 
The Ixodoidea, as far as is known, derive their nourishment entirely 
by sucking the blood of their hosts. These hosts are almost exclusively 
terrestrial vertebrates and include practically all classes of Mammalia, 
Aves, Reptilia (Chelonia, Lacertilia and Ophidia), and Amphibia ( Bafo), 
Two species are recorded as attacking Insecta ( Coleoptera )\ 
The Ixodoidea are distinguished from other Acarina by their 
relatively large size ; by the position of the spiracles (absent in the larva), 
situated as they are, in the posterior half of the body, behind the last 
pair of legs, or as in the Argasidae usually between the third and fourth 
pair of legs; and by the unique structure of the piercing and sucking 
mouth parts, complicated by the fusion of the basal segments of some of 
the perioral appendages so as to form a definite structure, the capitulum, 
articulating with the anterior end of the body. The body is compressed 
dorso-ventrally. The capitulum bears two pairs of jointed appendages. 
On the whole, the Ixodoidea are found to agree in their general morpho¬ 
logy with the remainder of the order Acarina, in so much as the 
cephalothorax is indistinguishably fused with the abdomen, in the posses¬ 
sion of an unsegmented body and in the character of the mouth parts. 
The Ixodidae resemble each other in all essential characters, but 
differ from the Argasidae in the fact that a hard dorsal shield or scutum, 
absent in the latter, is invariably present in all stages; also the capitulum 
is visible from the dorsal surface in the former, whereas in the latter it 
is entirely concealed (excepting in the larval stage) by the overhanging 
anterior extremity of the body: the two families differ also in many 
respects regarding their habits. 
The life-history of the Ixodidae may be briefly summarised as 
follows:— 
1 Neumann 1901, pp. 274 and 294. 
