1997 . The Journal of Arachnology 25 :321–325 MATING BEHAVIOR OF PHYSOLIMNESIA AUSTRALIS (ACARI, LIMNESIIDAE), A NON-PARASITIC , ROTIFER-EATING WATER MITE FROM AUSTRALIA Heather C . Proctor : Department of Biology, Queens University, Kingston, Ontari o K7L 3N6 Canad a ABSTRACT. The diversity of sperm transfer behavior shown by water mites (Acari, Hydrachnidia) i s among the highest in the Arthropoda . However, sperm transfer has been described in fewer than 10% o f water mite genera, all of them being Holarctic or cosmopolitan taxa . Here I describe mating behavior i n Physolimnesia australis (Halik 1940), the sole representative of an Australian genus . P. australis is unusual in having larvae that do not parasitize insects, and in including rotifers in its diet . The highly dimorphic P. australis male responds to female presence by taking up an "embrace" posture in which he orients hi s opisthosoma and legs III toward approaching females . The female is caught in the embrace and her leg s IV are secured by the modified tips of the males legs IV. The male deposits a glutinous mass on the females back, which she grooms towards her genital opening after being released . This mode of transfer differs from members of the confamilial genus Limnesia Koch 1836 in which males and females do no t pair. Chelicerates show the greatest diversity of venter copulation (e .g., Midea Bruzelius 1854 , sperm transfer modes in the Arthropoda. In Eylais Latreille 1796) to complete dissociation some taxa, males transfer sperm directly with in which the sexes never meet (e .g., Hydrod- a penis (Opiliones) or with secondarily de - roma Koch 1837) . Despite this amazing rang e rived genitalia (e .g., palps in Araneae) . In oth- of behavior, water mites have been poorly er groups, males transfer sperm indirectly by studied and mating observations have been depositing spermatophores on a substrate, and published for only 24 of the more than 340 either encouraging females to move over the genera of water mites (Proctor 1992a,b) . sperm packets (e .g., Scorpionida) or allowing These observations have been limited almos t females to discover and take up sperm on their entirely to species from North America and own (e .g., many Pseudoscorpionida) (Proctor Europe, and there are no descriptions of sperm et al . 1995). Finally, the horseshoe crab s transfer in a non-holarctic genus . Here I de- (Merostomata) have external fertilization of scribe mating behavior in a monotypic genu s eggs (Ruppert Barnes 1994) . Within the of Australian water mites together with casua l Chelicerata, some taxa exhibit greater behav- observations of its life cycle and diet. ioral diversity than others . For example, all spiders pair whereas pseudoscorpions may or METHOD S may not have close associations between th e Physolimnesia australis (Limnesiidae) is a sexes. The greatest variety of sperm transfer small (<_ 1 mm) water mite found in the lit- behavior occurs among the mites (Subclas s toral zone of standing and slowly running wa- Acari), and within this group the most diverse ter in Queensland and New South Wales (M behavior is shown by the water mites (Sub - . Harvey pers . comm. order Prostigmata, Hydrachnidia) . With the ; Proctor pers . obs.). This exception of external fertilization, all possible species shows a strong sexual dimorphism in modes of sperm transfer occur in the Hy- which males have a ventrally concave opis- drachnidia from direct transfer via venter-to - thosoma and flattened terminal segments o f legs III and IV (Fig. 1). Females are morpho- Current address: AES, Griffith University, Nathan logically similar to species in the confamilial 4111, Queensland, Australia. genus Limnesia and were previously described 321 322 THE JOURNAL OF ARACHNOLOGY as a species in this genus (Limnesia trituber- 1896, a cosmopolitan epizootic species of culata Viets 1955) . I collected and observed 200–380 µm in length (Koste Shiel 1987) . P. australis on two occasions in 1995 : in Thus to a 250 µm deutonymph, a single rotife r March from Cedar Creek approximately 50 would be a substantial meal . Adult male and km south of Brisbane, Queensland ; and in Oc- female P. australis were observed eating B. tober from a large pond on the University o f variabilis, as well as feeding on cladocerans Queensland campus, St. Lucia. All observa- and dipteran larvae . Neither the deutonymph s tions were made using a dissecting micro - nor the adults appeared to forcibly remove the scope and took place at the Department of En- rotifers from their moinid hosts ; rather, the tomology, University of Queensland . I mites captured rotifers that had detached fro m separated mites according to sex and stag e the cladocerans and were swimming freely i n (adult and deutonymph) and maintained thes e the wells . groups in large well plates (well diameter = Mating behavior .When a male P. aus- 35 mm, depth = 19 mm) . Dipteran larvae tralis was placed in a well that held females , (Culicidae, Chironomidae) and cladoceran s he initially stood on the substrate and (Moinidae) collected from a small pond on the groomed himself vigorously by moving leg s University of Queensland campus were in- III and IV back over his dorsum and around cluded as potential prey for the mites. I made to his venter. After a bout of grooming, the behavioral observations on mites that had male was very still relative to the females , been collected as adults as well as those raised which were constantly crawling and swim- from deutonymphs in the lab. Voucher speci- ming close to the substrate. In P . australis, as mens are deposited in the University o f in most linmesiids (pers . obs.), crawling lo- Queensland Insect Collection, Department of comotion is accomplished by the first three Entomology, St. Lucia, Australia, 4072 . pairs of legs, with legs IV moving in a con- stant fanning motion over the mites back, pre- RESULTS sumably aerating the dorsal integument fo r Life-cycle and predation.Most water gas exchange purposes . When a femal e mites have a complex life-cycle with three ac- bumped into the male or passed near him h e tive stages: the six-legged larva parasitizes immediately took up the "embrace" posture adult aquatic insects, and is followed by tw o (Fig. 1). In this position the males opistho- eight-legged predatory stages, the deuto- soma was tilted at approximately 30° to th e nymph and the adult (Smith Cook 1991) . substrate, the flattened tips of legs III were Physolimnesia australis is an exception to this touching and pressed against the substrate rule in that its larvae forgo the parasitic phase. (thereby forming the circular "embrace"), and Adult females collected from the field readily legs IV were held rigidly and vertically. The laid small clutches consisting of 1–8 eggs o n male oriented his embrace towards any fe- the sides and bottoms of the wells . The eggs males that passed behind him. He also orient- were large relative to the female (mean = 134 ed towards other passing males and occasion - µm, SD = 8 pm, n = 4). I observed that P. ally to swimming cladocerans . While in thi s australis larvae remain within the coating o f posture the male was often approached by a the egg clutch and transform directly into pre- female that -by accident or intent -crawled up daceous deutonymphs (via the inactive pro- behind the male and placed her capitulum tonymph). over the flattened tips of the males legs II I The newly emerged Physolimnesia deuto- (Fig. 2). The male responded by elevating his nymphs were very small (body length 25 0 opisthosoma to about 50° to the substrate and µm) and were unable to handle the large cla- attempting to capture the fanning tips of th e doceran and dipteran prey I provided. Never- females legs IV in the flattened, scoop-like theless, they increased in size and transforme d tips of his own fourth legs (Fig. 2). It wa s into adult mites . This energetic mystery was unclear how this capturing was achieved ; pos- solved when I observed deutonymphs captur- sibly, the long apical seta at the tip of the fe- ing and eating large phoretic rotifers that had males leg IV is secured by the groove in the been inadvertently introduced along with thei r males tarsus. moinid cladoceran hosts . The rotifers were When the male had captured both of th e identified as Brachionus variabilis Hempel females legs she typically began to struggle . PROCTOR—MATING BEHAVIOR OF PHYSOLIMNESIA 323 Figures 1—4 .—Mating behavior of Physolimnesia australis . 1, Male in the "embrace" posture wit h female approaching from behind ; 2, Female within males embrace, male has captured the tip of her left leg IV in the tip of his modified leg IV; 3, Male has captured both leg tips and the pair is swimmin g jerkily; 4, Female grooms sticky material deposited by male on her back towards her ventrally locate d genital aperture . 324 THE JOURNAL OF ARACHNOLOGY However, the male gripped the female in th e of Physolimnesia australis appears to fall be- region of her 2nd or 3rd coxal plates with th e tween the third and fourth categories, as the tips of his legs III (Fig . 3). The pair typically male places the sperm on the female (as in left the substrate at this point and swam abou t copulation), but she must move it to her gen- in a jerky fashion. I observed at least 20 pair- ital opening (as in pairing, indirect transfer) . ings that reached this stage; however, all but This suggests that the categories of sperm three were terminated when the female es- transfer outlined by Proctor (1992a) may be caped from the males grip after a few seconds too rigid to easily encompass all transfer be- of swimming .