EXTERNAL STRUCTURE : THE SCUTUM 573 



directed backwards, has attached to it a dorsal process (Nuttall and War- 

 burton) which consists of one or more teeth. The external article, 

 which may consist of three or more teeth, articulates with the outer 

 side of the internal article. The whole digit can be retracted or 

 extended by the action of the retractor and extensor muscles which are 

 attached to the bulb of the mandible. The teeth vary in size, shape and 

 number in the different species ; they are not of very great taxonomic 

 value. 



The radula, hypostome or labio-maxillary dart (Plate LXXIII, figs. 1, 

 2, etc. ; Plate LXXXV, figs. 6 and 7) is the most prominent structure 



on the ventral surface of the capitulum. It is an 

 , .,......,. . Hypostome 



elongate, spatulate piece of chitm arising from the basis 



capituli between the palps, and is composed of two symmetrical halves 

 which bear a number of small teeth with their points directed backwards; 

 these are arranged in rows, those on one half corresponding with those on 

 the other, forming regular transverse rows. The rows of teeth vary in 

 the different species, as regards the number and the size and shape of the 

 individual teeth. In many ticks the hypostome is armed with teeth from 

 its tip to its base, while in others they are only present on the distal two- 

 thirds, the remaining portion being smooth. The extremity of the 

 hypostome is often covered with a large number of minute teeth crowded 

 together in an irregular manner ; it is then spoken of as having a corona 

 (Nuttall and Warburton). 



In the male (Ixodini) the scutum covers the whole of the dorsal surface 

 (Plate LXXII, figs. 1 and 2), in the female only about the anterior half 

 (fig. 6). It is the chief characteristic of the Ixodini, and 

 is absent in the Argatini (Plate LXXII, fig. 5 ; Plate 

 LXXIV). The scutum is a hard chitinous plate of varying shape ; it may 

 be heart-shaped, oval, or more or less rounded (see figs.). Its anterior 

 end may be emarginate or deeply hollowed out ; the lateral angles in 

 certain species are prolonged into well-defined shoulders, the scapulae 

 (Plate LXXII, figs. 1 and 2), which project on each side of the basis capituli. 

 The scutum may be highly ornamented and the colouring is distributed 

 either in an irregular manner (Plate LXXII, fig. 2 ; Plates LXXIX 

 and LXXX), or collected together to form spots (Plate LXXII, fig. 6); 

 the surface of the scutum is always more or less punctate (Plate LXXII, 

 figs. 2 and 6), and may or may not possess hairs. The colour designs 

 and spots are well marked in certain species of Amblyoinma, Derma- 

 centor and Aponotnma (see figs.) but in the majority these designs should 

 not be entirely depended on for identification ; they vary in their 



