NO. 6 CHIGGER SUBGENUS GAHRLIEPIA — TRAUB AND MORROW 49 



discovery of the burrowing habit of G. penetrans, new species, particu- 

 lar attention was paid to examining the perinea of Borneo rats, squir- 

 rels, and other hosts by means of the stereoscopic microscope. Despite 

 the examination of 150 such mammals from Mari Parei, Tenompak, 

 and Ranau, no such overt lesions were observed and no further speci- 

 mens were collected other than the type series. 



G. penetrans, new species, exhibits some noteworthy morphological 

 modifications that are characteristic of the species and rare or absent 

 in other members of the subgenus. These are: (i) The body is quite 

 rounded, virtually as broad as long even when rather engorged. (2) 

 The eyes appear single and are contiguous with the scutum. (3) The 

 laterocaudal angles of the cheliceral bases are prominently developed. 

 (4) PW is two and a half times that of AW, instead of being about 

 twice that. (5) The ratio of PW to ASB is 6.8 instead of the usual 

 2-3 (with a maximum of 5) in other species. (6) The reduction in 

 the number and size of the dorsal setae. (7) Two-setose coxa III. 

 The first six of these modifications may be adaptations in connection 

 with, or as a result of, its burrowing habit. The stout body of this chig- 

 ger and its locus in superficial skin layers is reminiscent of the en- 

 gorged burrowing chigoe flea, Tunga penetrans. In each parasite, 

 advantage is in effect taken of .the physical principle that the ratio of 

 surface area to volume is a minimum in a sphere. In other words, a 

 spherical parasite packs the greatest possible volume in the smallest 

 possible area, thus mechanically traumatizing the host the least, pro- 

 vided the parasite is stationary when in situ, as in these instances. Cer- 

 tain of these modifications are expressions of, or are concomitant 

 with, the development of a spherical shape, i.e. (4) and (5), inasmuch 

 as a broad PM^ is to be expected in a stout species. Loss of bristles, 

 or reduction in their size, is characteristic of other ectoparasites with 

 a burrowing or semisedentary habit, as exemplified by the tungid and 

 hectopsyllid fleas. 



In many respects G. penetrans, new species, resembles G. ampiillata, 

 new species. Thus in both species the highly scrobiculate scutum vir- 

 tually covers the entire dorsum in unengorged specimens ; the body 

 is much more rounded than in other species ; the eyes are contiguous 

 with the scutum ; there is an intersensillary crater ; coxa III is 

 2-setose, and there has been a reduction in the number and size of 

 dorsal setae as compared with others in the subgenus. G. ampiillata 

 has been collected only twice, despite extensive trapping of the type 

 host in the type locality. It is possible that it, too, is a burrowing 

 species and hence has been overlooked. 



