The variation of the British harvest mite (Trombiculidae, Acarina)

Extract

Five main types of harvest mite larvae have been found in Great Britain. These may be distinguished according to the following key:

(1) Galeal setae ciliated

Galeal setae not ciliated (2)

(2) Scutal setae duplicated

Scutal setae not duplicated (3)

(3) Humeral setae duplicated Humeral setae not duplicated (4)

(4) Eight setae in both the second and

the third rows of dorsal setae Normal type a

Six setae in both the second and

third rows of dorsal setae Normal type b

Eight setae in the second dorsal

row but only six in the third Normal type c

Type a and type b may be further divided into groups based on differences in scutal width and setal length. Nevertheless, scutal width, setal length and setal number are not correlated with each other. Were larvae to be separated into two species on the hypothesis that one of these characters (e.g. differences in setal number) is genetically determined, then the range of variation with respect to the other two characters (namely scutal width and setal length) would be the same for each species. Forms with duplicated humeral setae do not differ otherwise from the normal type. The ciliation of the galeal setae and the duplication of the scutal setae is not consistent and larvae of mixed or intermediate type are known. It seems to me that these morphological differences may not be genetically determined but are solely phenotypic (and in the case of setal number may be influenced by rainfall and temperature). This, together with the lack of points of distinction among nymphs and adults, suggests that there is only one British species which was first described as Acarus autumnalis (Shaw, 17 90). As no type is known to exist, a lectotype has been selected from British material and deposited in the British Museum.

The systematic position of this species is difficult to determine. Until larvae of known ancestry can be reared under controlled conditions the systematic significance of the variations described above must remain in doubt.

Nymphs and adults do not appear to differ greatly from those described by André (1930). The adult is recorded for the first time in England. The male genitalia are described.