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VARIATION IN POPULATIONS OF L~ORATORY-REARED TROMBICULA (LEPTOTROMBIDIUM) AKAMUSHI (ACARINA TROMBICULIDAE) BY K. GoKsu, G. W. WHARTON and C. E. YuNIŒR. (Department of Zoology University of Maryland College Park). ' Individual variation is characteristic of all animais. The presence of this variation is one of the most perplexing and confusing phenomena that confronts the systematist in identifying and classifying' animais. The proper evaluation of variation is_ even more difficult in the case of sibling species. When these problems are associated with forms of public health signifi.cance, any information that may shed light on the extent and nature of variation is exigent. The classical vèctor of scrub typhus is Trombicula (Leptotrombidi~tm) akamushi (Brumpt, 9). Species of chiggers belonging to the subgenus Leptotrnbidit.tm are ali quite similar and are identifi.ed no the basis of key characùrs that literally amount to hair-splitting. For example, WoMERSLEY (952) in a key to members of this subgenus, differentiates T. (L.) palpalis (Nagayo et al., i99) froni fi.ve other species of the subgenus by "the presence of a branched ventral tibial seta, <J."rid makes further use of this character to separate other forms, including T. akamushi. Reports of morphological variation of chiggers are,commonplace ; WHARTON and FULLER (952) gives an extensive bibliography. However; 'these studies. are based mainly on specimens of unknown parentage. More recent exafuples of studies contàining remarks on variation are: WoMERSLEY (952), FuLLER (!952), GouLD (954); TRAuB and MoRRow (955), FARRELL (956), TRAUB ànd EvANS (-957) and SASA (958). In the present study, collections of laboratory-reared T. aka:. mushi, consisting of families from specifie matings, were ahalyzed in order to obtain an estimate of the amount and nature of variation that could be anticipated among sl.blings.. Present address : University of Ankara, Department o.f Protozoology and Arthropodology, Veterinary Faculty, Ankara, Turkey. -.. Acarologia, t. II, fasc. 2, 96.
' - MATERIALS AND M,ÈTHODS. One hundred and ninety-six specimens of Trombicula (Leptotrombidiumj akam2tshi, th~ offspring of II different mated pairs of laboratory-reared adults, were obtained from the Departmenf of Entomology of the Walter Reed Army Institute of Research. The adults were obtained as larvae from Rattus rattus jalorensis (Bonhote) trapped in the vicinity of Subong, Selangor. State, Malaya, August, 948. They were subsequently sent -to Walter Reed Army Institute of Research by Dr.). R. AuDY and Lt. Col. R. TRAUB. Laboratory rearing: of the parents and isolation of larvae were done by Mr.-Louis J. LIPOVSKY and Mr. T. J. NEAL (W. R. A. I. R.).. The material was then turned over to us for systematic --:- 2 -;- study through the courtesy of Maj. H. BARNETT (W. R. A. I. R.). Ali larvae were -examined with the aid of phase-contrast microscopy. Mor phological variations were noted and drawn,, and sorne. photomicrographs were made. Tables were prepared enumera ting the variations.. Statistical tests for the analysis of variance within and among families were performed for a -morphological character that showed wide variation. DESCRIPTION OF LARVAL T. Akamushi. The generalized description that follows is based on sp~cimens of this study. Bo~y : Ovate and striate, averaging r6o fj. wide by r8o fj. long. Eyes- paired, on ocuhi.r plate situated posterolaterally of and adjacent to scutum ; anterior pair at level of pseudostigmata, slightly larger _and better developed ;' posterior pair just posterior to posterolateral set~e. Scutum -:- nearly rectangular, anterior margin convex in center, anterolater;:tl corners rounded or slightly expànded; posterior margin sinuous, slightly concave medially, slightly convex submedially, terminating in anu~ar, bulbous, posterolateral corners ; lateral margins slightly ' concave. Scutum with five plumose setae, two branched fl.agelliform sensillae arising from pseudostigmata, and numerous punctae. Anus - a small longitu _dinal opening located in midline of venter at level of sixth row of ventral setae. Body setae - all.plumosë and similar to ~cutal setae; dorsally composed of 3-38 sejae arranged in six to seven rows ; number and arrangement of rows variable (2-8-6-6-8-3 ;?-8-6-8-6-2 ; 2~ro-8-6-5-2 ; 2-g-8-7-6-5-; etc.) ; ventrally are 24-33 slightly smaller setae, c6mprising seven to eight rows; number and arrangement variable (2-2-4-6-4-4~3; 2-2-4-5-4-4-2-2; 2-2-5-7-4-4-2 ; 2-2-4-6-6-4-4;. etc.). Gnathosoma : Chelicerae - basal segment punctate, generally rounded posterolaterally, but sometimes sharply angulate ; distal segment with a single subter- I. The authors are grateful to the above mentioned persans for their efforts ; this study was aided by support from the National Science -Foundation, Grant No.. NSF-G 456.
-2- minai tooth; pseudochelae may or may not be visible. Palps - fused coxa and trochanter punctate, coxal seta branched; single dorsal seta of femur and genu nude; one branched dorsal, one nude lateral and one nude ventral seta on tibia; palpai claw trifurcate, accessory prongs lateral to and smaller than axial prong; tarsus with five to seven (usually six) branêhed setae and a single striated spur. Galea - with a single branched seta. Legs : All legs with seven punctate segments terminating in two claws and a clawlike empodium; all coxae and trochanters each with a single branched.seta. Leg I - basifemur with a single branched seta-; telofemur with five brariched setae; genu with four. bra!lched and two nude setae (genualae) arid a microgenuala; tibia with eight branched setae, two tibialae and a microtibiala; tarsus with 7-2 branched setae, a blunt striated sensory seta (spur), a microspur, a pretarsala, a subterminala and a parasubterminala. Leg II - basifemur with two branched setae; telofemur with four branched setae; genu with three branched setae and a genuala ; tibia with s~x branched setae and two tibialae ; tarsus with about 3 branched setae, a spur and 'a microspur and a pretarsala. Leg III - basifemur with two branched setae; telofemur with three branched setae ;' genu with three branched setae and genuala ; tibi~ with six branched setae and a tibiala ; tarsus with about 2 branched setae; claws and empodium longer and narrower than in legs I -and II. DESGRIPTION OF MORPHOLOGICAL VARIATIONS. Variations were noted in scutal shape, number and arrangement of body setae, shape olthe basal segment of the chelicerae, and-setation of the palps and legs. Scutum - The_ scutum is normally semi-rectangular, with a convex anteromedian margin and a sinuous posterior margin. Variations in the shape of the anterior scutal margin were -from flat or nearly flat (49 specimens) to convex or slightly convex (45 specimens); a single specimen showed a concave anterior margin, and another was undeterminable. One hundred and eightythree specimens exhibited a sinuous posterior margin, while twelve others varied from concave tb convex. The scutum of one spec!men was undeterminable. Variation in the shape of the scutum may result from preparation of the specimen for study as well as from actual differenèes in form. For this reason standard measurements. of the scuta were not studied. -. Anomalies noted in morphology of thè scutum were confined to setal configurations. One larva showed two anterolateral scutal setae on the -right _side instead of the normal one (:f>l. I, Fig. 2), while another larva possessed only a short rudiment of this seta (Pl. I, Fig. ). One specimen exhibited a malformed.scutal seta in this position, in which the shaft of the setét bore a unique projection (PLI, fig. 3). A duplicated left posterolateral sefa was present in a single specimen (Pl. I, Fig. 5).
' /' Anomalies and variations in morphology of Trombicula (Leptotrombidium) akamushi,,!, FIG. r. - Underdeveloped anterolateral scutal seta. :_ FIG. 2.. - Double anterolateral scutal setae.- FIG: 3 -Unique projection of anterolateral scutal seta.- FIG. 4 - U~Jder7 developed seta on trochanter III. - Fiq. 5 - Double posterolatèral scutal setae. -:-; FIG. 6. - ' Pseudochela. - FIG. i - Branched ventral seta of palpal tibia. :_ FIG. 8. -: A single tibiala I. - FIG. g. - A single tibiala II. '
23- Body setae- Considerabie differences were observed in 'the number and arrangèment of body setae. Dorsally the number ranged from 28 to 45, with an average of 36 (Table I). Dorsal rows were six or seven in number, and showed no consistent composition. Similar variation was seen ventrally. Ventral setae ranged from 3 4 Variations in morphology of Trombicula (Leptotrombidium) akamùshi. FrG. I. - Three tibialae I. - z. Three genualae I. - 3 Two genualae II. 4 Three tibialae II. - 5 Branched seta of palpai femur. 22 to 38 with an average of 28, comprising seven ç>r eight rows, each of variable composition.. Gnathosoma- Pseudochelae (Pl. I, Fig. 6) were identifiable in only four specimens, but their absence could often not be confirmed in the remaining specimens due to their position and hyaline nature. The posterolateral. outline of the basal
-24-. segment?f the cheliceni. was round or neariy round (in 32 specimens) but varied from slightly angular (in 32 specimens) tà sharply angular (in 32 specimens) (Table II). The ventral seta of the palpai tibia was branched in 68 out of rg6 specimens (Pl. I, Fig. 7). The branched -setae of the palpai tarsus were six in number in 72 specimens, while 5 specimens had five branched setae and nine specimens had seven branched setae (Table II). A rare variation was a branch on the otherwise nude dorsal seta of the palpai femur, which occurre~ in one specimen (Pl. II, Fig. 5) _- Legs - I : The genu of two larvae had three rather thari two genualae, while other setae of this segment were normal (Pl. II, Fig. 2). The tibia of one specimen had three instead of two tibialae (Pl. II, Fig. r), whilê that of another specimen had only one tibiala (Pl. I, Fig. 8) (in both of these, other setae of the segment were normal). The branched setae of the tarsus ranged from r6 to 22 in number (Table III). II : A single trochanter of one specimen lacked seta and setal base, although it was present on the opposite member. Setae of the genu varied from the normal pattern (three branched and one nude) as follows : two branched and two nude (one specimen) (Pl. II, Fig. 3), three_ branched and no nude (one speèimen) four brançhed and one nude (one specimen). The tibia of a single specimen had a single tibiala (Pl.), Fig. g), while that in, another specimen had three tibialae, as opposed to the normal two (Pl. II, F{g. 4). The branched setae of tarsus II varied from I2 to I5 (Table III). III : In a single specimen the seta of the trochanter was seen to be reduced on the right member,. being short, tapering and unbranched (Pl. I, Fig. 4). The branched setae of tarsus III varied from II' to 4 (Table III).... An analysis o'f group me ans (Table IV) was applied to the families of chiggers using the number of dorsal setae (Table I) as a variable characteristic within and among families. This gave a significant value at the. Ol_level of significance. DISCUSSION. Variations in morphology of Trombicula (Leptotrombidium) akamushi were noted from individual to individual within the families studied, and from family to family within the populations studied. These variations ofte:t:j. concerned characters considered by specialists to be useful taxonomie attributes. One of them (averagè number of dorsal body setae) was shown to vary from family to family significantly more_ than within single families (Table IV).. Since ail of the familles were reâred under similar conditions, genetic rather than environmental influence was probably responsible in this instance. _ Variations is more< apparent in popùlations living in controlled environments. This phenomenon is so weil recognized that Darwin believed domestication induced variation. However, this view- has generàlly given way to the beliet' that devia-
- -~ - -25- ting types in the field have less chance for survival; and therefore are less likely to be found (MÜNTZING, I959) N evertheless, the frequency with which variation occurred in the laboratory-reared groups of siblings under study should serve to emphasize that variants will be found in the field. Furthermore, the finding that at least sorne of these variations are inherited indicates that whole populations may differ from each other to the same degree that two siblings might differ. Such populations have in fact been described (WoMERSLEY, r944). Table I NUMBER OF DORSAL SETAE OF 96 Trombicula akamushi REPRESENTING ELEVEN FAMILlES. Family No. I 2 3 4 5 6 --- 7 ' 8 9 IO II Total 37 36 34 3 29. 34 34 36 38 38 35 37 44 32 34 3 36 34 37 35 34 43 37 36 35 3 33,32 33 4 38 35 38 37 35 32 33 32 32 34 36 36 35 38 4 39 3 32 29 33 36 39 38 38 36 36 3 32 3 39 36 37. 4 38 37 33 32 3 44 34 37 37 4 33 37 3 34 4 37 35 38 33 3 36 36 38 36 37 37 32 33 32 37 44 39 37 39 35 32 32 38 39 35 36 38 3 32. 33 34 4 37 36 37 3,3 32 38 34 37 38 38 28 3. 32 39 38 4 38 37 3 32 32 35 4 4 35 44 35 3 3 35 44 36 39 37 32 33 3 35 38 36 39 3I 32 32 34 45 38 38 3:3 34 3 36 37 38 37 3 32 3 4 38 38 38 32. 32 33 38 38 33 29 33 39 4I 3 32 36 37 34 3 3 33 33 28 3 29 33. n 24 24 29 2 26 2 5 2 6 4 6 96 x 37-2 38. 3I.8 32.2 3.5 36.2 34-4 38.8 37 3 35 5 38 7 35.6
Q Table II GNATHOSOMAL VARIATION WITHIN AND AMONG ELEVEN FAMILlES OF CHIGGERS. Family No. - No. of specimens showing No. of specimens showing i No. of specimens with 5, posterolateral cheliceral ventral seta of palpai No. m argin : t ibia : 6 or 7 branched setae on palpai tarsus : of Specimens rounded angulate nude branched 5 ' 6 7 I 24 22 2 I7 7 I 23 2 24 I3 II I5 9 I 23 ' 3 29 26 3 23 6 29 4 2I I5 6 I5 6 2I 5 26 I9 7 r8 8 I 22 3 6 2I I2 9 IO II 6 I5 ' 7 5 4 I 4 I 3 2 ' 8 2 4 I6 I2 8 2 I4 4.. 9 r6 9 7 II 5 I I3 2 IO - 4 3 I 4 v 4 II 6 5 I 3 3 6 Total I96 I32 64 I28 68 I5 I72 9 ' - - -- -
:... ~ "" ~ " :-'" _...... rf p.!"... "'..? F am r y [ f No. S Table III TARSAL SETAL VARIATON WITHIN AND AMONG ELEVEN FAMILlES OF CHIGGERS. Tarsus I Tarsus II Tarsus III No. ~im::;:~_l_: :: ~- --"'-- ~ _:!_: "3_ ~: "3 xx l_:_l ~ I4 I 24 7 II 4 2 5 I9 I4 9 I 2 24 7 I2 5 9 I3 2 II I2 I 29 6 8 IO 4 I 8 I5 6!li 5 6 2 7 3 8 4 IO II 9 4 2I 26 2I 5 2 I6 4 6. 4 5 9 3 4 I IO 3 3 5 IO I 2 4 3 I 2 2 I 2 4 7 3 3 2 6 2 2 3 3 2 I Total l 96 46 6I 5I I6 I5 I I 3 2 2I 3 3 2 5 I2 I 3 IO 8 2 5 I2 5 4 3 I6 7 7 3 7 2 2 9 7 I 3 2 3 2 9 5 4 7 9 3 I 9 3 3 5 IO 2 I I I 2 I o 2 3 o I 'l o 4 2 o 2 --;- ----;- ---;- ~--;- --;----;- ~.;. Undeterminable on tarsus II of r specimen. 2. Undeterminable on ali tar~i of 2 specimens. 3 Undeterminable on t arsus III of ' 2 specimens. 4 Undeterminable on t a rsus III of r specimen.
-. - zo8 The taxonomist then, in constructing keys and diagnoses, as weil as in describing forms, should beware of relying wholly upon characters capable of variation. Thus, 68 out of rg6 spe~imens of T. (L.) akam~tshi in this study would not key out properly in WŒ\ŒRSL;EY's (rg5z) key because a couplet is based solely i.ipon whether ventral seta of the palpai tibia is branèhed or nude (P. 33 : couplet g). Similarly, SASA (958) classified the leg setal patterns of the supraspecific groups of the Trombiculidae. He ' found that the subgenus Leptotrombidùtm contained species having essentially similar chaetotaxy. To accept such a diagnosis per se would lead one into taxonomie difficulties when confronted with such forms as the _variants in this study. The extreme amount of variation observed in the present small sample, even among siblings, suggests that future studies of trombiculid systematics should include laboratory rearing of larvae, expecially in the case of medically important species. Table IV ANALYSIS OF VARIANCE : NUMBER OF DORSAL BODY SETAE PER INDIVIDUAL WITHIN AND AMONG II FAMILlES OF Trombicula akamushi. Source of Variance Degrees of Freedom Among families.. Within familles.. 85 Total... 95 Sum of Squares '57.9 834 33 Mean Square -. -- 57.9 4 5 F LITERA TURE CITED ' FARRELL (C. E.), 956.- Chiggers of the genus Euschongastia (Acarina : Trombiculidae) in North AmeriCa. Froc. U. S. Nat. Mus. Io6 : 85-235. FuLLER (H. S.), 952. -The mite larvae of the family Trombic~lidae in the Oudemans collection : taxonomy and medical importance. Zool. Verhandel. I8 : r-26r. GouLD (D. J.), 956. -The larval trombiculid mites of California (Acarina : Trombicu- ' lidae). Univ. California Publ. Eitomol. II : -rr6 MüNTZING (A.), 959 - Darwin's views on variation,under domestication. American Sei. 47 : 34-325. SASA (M.), 958. - Comparative studies on the leg chaetotaxy of larval trombiculid mites of Japan. Japanese ]oum. Exper. Med. 28 : II-34 TRAUB (R.) and EVANS (T. M.), 957.- Records and descriptions of-chiggers of the subgenus Walchia Ewing, 93, from Southeast Asia (Acarina, Trombiculidae). Malaysian Parasites 28 : 297-358. ' r. Significant at % level of significance.
-zog- TRAUB (R.) and MoRROW (M. L.), 955. - A revision of the chiggers of the subgénus Gahrliepia (Acarina : Trombiculidae). Smithsonian Mise. Coll. I28 (6) : 8g pp. WHARTON, (G. W.) and FuLLER (H. S.), rgsz.- A manual of the chiggers. Mem. Entamol. Soc. W asghinton 4 : r85 pp. WOMERSLEY (H.), rg44. :- Notes on and additions to the Trombiculinae and Leeuwenhockiinae (Acarina) of Australia a~d New Guinea. Trans. Ray Soc. S. A2tstralia 68 : 8z-rrz. WoMERSLEY (H.), rgsz. -The scrub-typhus and scrub-itch mites (Trombiculidae, Acarina) of the Asiatic-Pacifi.c region. Rec. S. Australian Mus. Io : 673 pp. -