394 



BUREAU OF ANIMAL INDUSTRY. 



Species based upon color alone must, therefore, usually be viewed as 

 probably invalid. 



On the body may be recognized dorsal surface^ ventral surface^ sides^ 

 or lateral margins, and anterior and posterior margins^ all of which 



possess structures which are of system- 

 atic importance. 



A. Dorsal structures. — On the dorsal 

 surface are found: Scutum {(iorsdX shield), 

 eyes^ dorso-suhmedian porose plates^ pos- 

 tero-marginal festoons and shields, fur- 

 rows^ pits, punctations, and hairs. 



(1) The scutum.^ or dorsal shield (also 

 called thoracic shield, dorsal plate, occa- 

 sionally erroneously called the head), 

 characteristic for the family Ixodidse, but 

 not found in the family Argasidge, is a 

 hard leather-like plate on the back. In 

 the male (fig. 43) it usually covers the 

 entire or almost the entire dorsal surface; 

 in nymphs (fig. 47), it covers the anterior 

 portion; and, on account of its size in 

 proportion to the size of the body, 

 nymphs are occasionally mistaken for 

 males. In the adult female (fig. 43), it is 

 proportionately much smaller and con- 

 fined to the anterior end of the body. 

 Usually it is more or less distinctly di- 

 vided by two longitudinal cervical grooves^ 

 which are of some systematic importance. 

 Its anterior portion may be more or less 

 distinctly emarginate to receive the base 

 of the capitulum, in which case one or 

 two pairs of antero-terminal prolonga- 

 tions (fig. 60) of the scutum are present. 

 The scutum may bear coarse or fine pores 

 (punctations) with or without hairs; it 

 may be quite uniform in color, or may be 

 more or less highly colored by the pres- 

 ence of a rust of metallic luster, giving 

 rise to various designs. The latter are 

 subject to great variation, even among different members of the same 

 species, and while they are often useful in aiding in the recognition of 

 a species, they can not be admitted as of prime specific value. Under 

 no circumstance, so far as can be seen at present, should these color 

 designs alone be taken as basis for establishing new species. 



Fig. 65.— Leg I of spinose nymph of Or- 

 nithodoros Megnini. Greatly enlarged. 

 Original. 



