TAXONOMY OF TROMBICULIDS 
I3i 
4. The structure of the scutum (crista), Nymphs) Adults. — There appears to be no satisfactory 
account of the detailed structure of the scutum, and artists drawing these structures have 
sometimes shown only one plane, so that similar scuta may be given very different appearances 
by different artists. The artist himself is seriously handicapped by the lack of a comparative 
study of this structure. The present writer suspects that there are some fundamental 
differences in the structure of the scuta of certain groups of trombiculids. A comparative 
study of uncrushed material as well as serial sections is likely to yield some valuable taxonomic 
clues. The features which will most repay investigation appear to be (a) the structure of the 
chitinous vault or roof (areola) of the area sensiligera, between the sensillary bases, (b) the 
development and structure of the apodeme beneath the scutum, (c) the shape of the space 
enclosed by the walls of the apodeme beneath the area sensiligera (this space may extend 
towards the chitinous column of the crista), and (d) the development of the scutal structures 
from nymph to adult. The eyes, the para-scutal setae, and the tectum with its seta(e) should 
be studied together with the scutum. 
The larval stage* 
1. Standard Measurements. — A comparison of standard measurements of the scuta can be 
misleading and the ready separation of different populations of chiggers by statistical analysis 
of their measurements is a new pitfall for the unwary. Ratios are generally more valuable than 
linear measurements. Superimposing tracings of scuta, drawn to the same scale, is a useful 
way of deciding which ratios are likely to help in separating particular species. Ratios between 
scutal measurements and measurements of other chitinised parts may be particularly valuable. 
The present writer has been measuring the tarsus-plus-pretarsus for other reasons and would 
prefer to take scutal/tarsal ratios. Traub has shown that a scutal/coxal ratio (PW: coxa II) 
is of great value in separating T. akamushi from T. deliensis (Traub & Audy, 1953: 86). 
Jadin & Vercammen-Grandjean (1952) have added to Womersley’s standard scutal 
measurements but the writer has no comment to make on these except to plead that the number 
of measurements should be cut down to the absolute minimum. Only usage and experience 
will allow us to decide what the minimum should be. Certain measurements are useful in 
some groups but not in others ; a routine tabulation will therefore encourage either superfluous 
measurements or unfortunate omissions. 
2. Chaetotaxy. — Wharton (1948) found that a detailed study of the chaetotaxyof the legs 
was of assistance in the recognition and classification of trombiculid larvae, and the importance 
of this finding has been conclusively demonstrated since. Brennan & Wharton (1950) have 
surveyed the subgenus Neotrombicula in North America and have shown that the leg setae are of 
specific importance and can be used to advantage in constructing keys. More recently, 
Brennan (1952) has done the same with the four known American members of the T. cynos 
species-group, while Audy (1952) has dealt with a mixed collection of species of Trombicula 
from bats in Malaya. During the course of studies of Malayan material, in which comparisons 
have been made with the observations of Wharton and of Brennan, some ideas have emerged 
concerning the importance of various details, and these are noted below. 
* Since this paper went to press, P. H. Vercammen-Grandjean, of the Laboratoire Medical, Province of 
Kivu, Bukavu, Belgian Congo, has made a most useful observation of a taxonomic character which has 
hitherto escaped attention, namely, the nature of the internal annular chitinous thickenings of tarsi I in the 
larvae. These may be partially or fully developed, and there may be one (proximal) or two (proximal and 
distal). A detailed assessment of this character is being made the subject of a joint paper by Vercammen- 
Grandjean and Audy, but we may here note that the second distal thickening appears to be regularly 
present in the Gahrliepiines, in the species of Walchiella and the lacunosa-group so far examined ; while it is 
poorly developed in some members of the indica-gvowp. It is present in a few members of the subgenus 
Trombicula, and is clearly shown in Lipovsky’s drawings of Speleocola.lt is however absent in most chiggers. 
MALAYA, No. 26, 1953 
