Mite morphology

 

Overview of diagnostic characters

Since bee-associated mites come from a variety of diverse lineages (see tool scope and classification), it is difficult to provide an exhaustive morphological account. Please consult Krantz & Walter (2009)) for additional details. Here we offer a short introduction to mite morphology, focusing on diagnostic characters for the three principal groups that include bee-associated mites: Mesostigmata, Astigmata, and Prostigmata. In the comparison table below, an asterisk indicates a defining trait for that group; if a mite bears that trait, it is undoubtedly a member of that group, and checking other characters is not necessary. Other rarely used diagnostic traits, such as those used to distinguish two or a few mite genera are pointed out in fact sheets and key images.

See the "Setae, solenidia, & suckers" tab for more details on using these features for mite identification.

Mesostigmata Astigmata Prostigmata
tritosternum *present (Fig. 1) absent absent
leg coxae *free, not fused to body (Fig. 1) fused to body, form distinct coxal apodemes (Fig. 4) fused to body, form distinct coxal apodemes (fig. 7)
stigmatic openings *present on mid sides of the body, posterior to leg II or III insertions (Fig. 1) *absent *present, either near associated with the chelicerae or on the dorsal propodosoma (Fig. 7)
empodial claws; lateral claws empodial claws absent from all legs, lateral claws usually present, at least on some legs *only empodial claw is present (Figs. 4, and 6); lateral claws are absent from all legs lateral and empodial claws are usually present, at least on some legs
male aedeagus absent; some have spermatodactyl instead (Figs. 2, 3) present (Fig. 5) present or absent
dispersal stage/non-dispersal stage dispersal stages (fertilized female or deutonymphs) are not substantially different from non-dispersal stages *dispersal stage (phoretic deutonymphs) substantially different from non-dispersal stages (Fig. 6) dispersal stage is not substantially different from non-dispersal stages
attachment organ absent present (Fig. 6) absent
chelicerae in dispersal stage present absent (Fig. 6) developed but sometimes indistinct
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Fig. 1 Mesostigmata
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Fig. 2 Mesostigmata
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Fig. 3 Mesostigmata
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Fig. 4 Astigmata
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Fig. 5 Astigmata
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Fig. 6 Astigmata
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Fig. 7 Prostigmata

Setae, solenidia, and suckers

Setae, solinidia, and suckers are morphological features frequently used for identification of mite groupings (e.g., families or genera). Below are listed individual such terms relevant for identification of the set of bee-associated mites covered in this tool.

Coxal setae are paired setae situated on or near coxae.

coxal seta 1a: situated on coxae I. In Astigmata present in all active developmental stages. In phoretic deutonymphs of Astigmata, can be modified to a conoid or be vestigial or absent.

coxal seta 3a: situated on coxae III. In Astigmata present in all active developmental stages. In phoretic deutonymphs of Astigmata, can be modified to a conoid or be vestigial or absent.

coxal seta 4a: situated on coxae IV. In Astigmata appear at the stage of deutonymph. In phoretic deutonymphs of Astigmata, can be modified to a conoid or be vestigial or absent.

coxal seta 4b: usually situated between setae 3a and anterior to setae 4a or occasionally absent. In Astigmata appear at the stage of deutonymph.

genital seta g: Paired setae associated with the genital region. In Astigmata appear at the protonymph stage.

external scapular seta: Paired prodorsal setae se in Astigmata (also known as sce; in Oribatida known as ex).

external vertical seta: Paired prodorsal setae ve in Astigmata (in Oribatida known as le); sometimes absent.

internal scapular seta: Paired prodorsal setae si in Astigmata (also known as sce; in Oribatida known as in).

internal vertical seta: Paired prodorsal setae vi in Astigmata (in Oribatida known as ro).

supracoxal seta: Paired, often modified dorsal coxal seta (scx) in Astigmata associated with the supracoxal gland opening. Situated on the lateral propodosoma (because coxa are fused to the body in Acariformes). Also known as elc I.

postanal seta: Unpaired median seta inserted posterior to the anal opening in the Mesostigmata (Parasitiformes); usually on the anal or ventrianal shield.

In the Acariformes, in the ancestral complement of five setae surrounding the empodial claw ventrally, one is unpaired and the most ventral (s), and four others are paired (u' and u'' and p' and p''). Relative to each other, the unguinal setae u are more ventral and the proral setae p are more dorsal.

proral seta p or proral p: Paired disto-ventral setae (p' and p'') on tarsi I-IV of Acariformes.

subunguinal seta s: Unpaired disto-ventral seta on tarsi I-IV of Acariformes.

unguinal seta u: Paired disto-venrtal setae u' and u'' on tarsi I-IV of Acariformes.

tarsal seta aa: On tarsus I in some Astigmata. Usually situated close to solenidion ω2 in basal part of tarsus. Absent from tarsus II, hence can be easily identified by comparison of tarsi I and II setations. Homologous to anterolateral setae a'' of Oribatida. (Not illustrated.)

tarsal seta ba: On tarsi I-II in many Astigmata. Usually situated in the mid dorso-anterior part of tarsus Many Acaridae lack ba from tarsus I in phoretic deutonymphs, while this seta is present in adults and other feeding stages (in contrast, setae ba II uniformly present on tarsi II in all stages of these taxa). Homologous to fastigial setae ft' of Oribatida. (Not illustrated.)

tarsal seta e: Posterodistal seta on tarsi I-IV of Astigmata. On tarsi I-II of phoretic deutonymphs of Acaridae and related lineages (Astigmata), these setae have a characteristic 'saucer' at the tip. Homologous to tectal seta tc'' of Oribatida.

tibial seta gT: Anterior seta on tibiae I-II of Astigmata. Homologous to seta v' of Oribatida. (Not illustrated.)

tibial seta hT: Posterior seta on tibiae I-II of Astigmata. Homologous to seta c'' of Oribatida. (Not illustrated.)

tibial seta kT: Anterior seta on tibiae III-IV of Astigmata. Homologous to seta v' of Oribatida.

Solenidia ω1, ω2, and ω3 are found on tarsi in many Acariformes.

solenidion ω1: In Astigmata, usually situated in the dorso-basal part of tarsi I-II (absent from tarsi III-IV) and typically has a club-shaped, or rounded terminal end. Invariably present in all developmental stages.

solenidion ω2: In Astigmata, usually situated in the basal, posterolateral part of tarsus I (and absent from all other tarsi), often close to setae aa. Often is much shorter than ω1 and has a rounded tip. Present in protonymphs to adults and absent in larvae.

solenidion ω3: In Astigmata, usually situated in the anterio-lateral part of tarsus I (and absent from all other tarsi). In phoretic deutonymphs usually situated in the basal part of tarsus (sometimes close to ω1), while in adults and other feeding stages it is terminal. Present in deutonymphs to adults and absent in larvae and protonymphs.

Suckers ad1+2 and ad3 and conoids ps1 and ps2 are paired suckers of setal origin located on the attachment organ of phoretic deutonymphs of Astigmata. Suckers function as attachment structures by creating negative pressure; they are modified from setal alveoli, vestiges of which are visible in the center of the sucker. In contrast, conoids are attachment elements that create adhesive forces; they are modified setae arising from well developed alveoli.

sucker ad1+2: Paired true suckers (from adanal setae ad1 and ad2 fused together) typically situated centrally on the attachment organ. Each sucker has two setal alveoli (often fused into a single alveolus).

sucker ad3: Paired true suckers (from adanal setae ad1 and ad2 fused together) typically situated anteriorly on the attachment organ. Each sucker has a setal alveolus.

conoid ps1: Paired conoids originating from pseudanal setae 1 (ps1) typically situated posterolaterally on the attachment organ. Lacks setal alveoli.

conoid ps2: Paired conoids originating from pseudanal setae 2 (ps2) typically situated medio-laterally on the attachment organ. Lacks setal alveoli.

tarsal sucker: In some genera of Astigmata, tarsal setae e (=tc'') and d (=ft'') that are modified to suckers on tarsus IV of the male. Used for holding female during copulation. Otherwise, these setae are filiform and not modified.

Adult sex determination

Adult female mites have an ovipore, while adult males have an aedeagus (Astigmata [Fig. 2] and some Prostigmata; see Overview), a gonopode (Mesostigmata [Fig. 1]), or a spermatopositor (Oribatida and some Prostigmata).

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Fig. 1. Ventral view of female and male of Proctolaelaps longanalis (Parasitiformes: Mesostigmata), showing sexual dimorphism in the degree of development and fusion of ventral shields, positions of genital opening, and the general morphology of chelicerae.
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Fig. 2. Ventral view of female and male of Chaetodactylus micheneri (Acariformes: Astigmata), showing prominent sexual dimorphism in the position of the structures related to copulation and reproduction (ovipore and copulatory opening in the female and genital apparatus, including aedeagus, in the male).

Developing eggs can be observed inside females. Eggs are usually large compared to the female body and are easily noticeable. In addition to the ovipore, mite females in Astigmata and Mesostigmata may have a copulatory opening. These openings serve for insemination, are usually very small (Fig. 2), and their position varies in different groups of mites.

Sex is usually apparent only in adult mites. Locustacarus is an exception. In this species, the larval stage is represented by female larvae (called "larviform females"), which can copulate but lay eggs after molting, and larviform males, which do not molt further. Sometimes, despite lacking functional genital structures, immature stages can be sexed based on differences in shape and size, since males are usually smaller.