Opiliones (Harvestmen) Large chelicerae useful for identification Leg-like pedipalps of various morphologies, useful for identification Odoriferous gland for defense pruposes 2 median eyes sitting on an ocularium Prosoma & opisthosoma broadly joined, often merged together Dorsal side often covered by a +/- long scutum (shield) Legs often elongated, with tarsus split in numerous tarsomeres (up to 100) Anus often can be way more forward than in other arachnids Genital opening thrust forward between the legs and also bears the tracheal respiratory system Legs’ coxae well-developed & forming a stomotheca (a pre-oral chamber before the mouth) Note: Harvestmen show a wide diversity of morphologies concerning the prosoma-opisthosoma fusion and their ventral side, so the one here doesn’t exist & is a fusion of several types to reflect the broad average of what you can find among these arthropods.
Palpigradi Large projected forward chelicerae No eyes, presence of specific sensory organs at the body front (called “frontal organ” & “lateral organ”) Prosomal carapace subdivided from front to back into: propeltidium, mesopeltidium, metapeltidium Last pair of legs 8-segmented Last 3 segments of the opisthosoma smaller & forming a postabdomen Telson modified into a flagellum of 15-ish segments, can easily break Ventral sacs (if Eukoeneniidae, absent in Prokoeneniidae). No external respiratory system, the animal is small enough for gas exchanges to occur through the weakly sclerotinized exoskeleton Genital plate extending backward Long pedipalps, used for walking, retention of 3 claws at the appendage’s end The first pair of legs is 12-segmented and mainly used as sensory appendages while walking
Solifugae 2-segmented super enlarged chelicerae 2 median eyes on a high spot Prosomal carapace subdivided from front to back into: propeltidium (larger & raised), mesopeltidium, metapeltidium Patella is usually longer than the femur Tarsus divided into tarsomeres, usually 3 tarsomeres but sometimes more (like 6 among solpugids) Double trochanter on the 3° & 4° walking legs Soft 10-segmented opisthosoma 3 pairs of tracheal openings on the underside of the opisthosoma malleoli/racquet organs on the coxae & trochanter of the last pair of legs, acting as sensitive organs close to the ground 1 pair of tracheal openings between the 2° & 3° walking legs First pair of walking legs significantly shorter than pedipalps Long & strong leg-like pedipalps, covered with highly sensitive setae Adhesive organ at pedipalps’ tip
The mite body plan Although not forming a single monophyletic group and having a morphological diversity way too vast to be exhaustively synthesised in a single drawing, all chelicerates called “mites”, from flour mites to ticks, share a broad body plan easily recognizable, even tho there are many variations depending on which group you’re looking at. The prosoma’s part bearing chelicerae & pedipalps is detached from the body and is called “gnathosoma” Prosoma & opisthosoma fused, forming a new body part called “idiosoma”. Some groups have dorsal shields. The front half bearing the first 2 pairs of legs is called “propodosoma”, and the back half “hysterosoma) Small eyes can be present, most often on the idiosoma forward sides Legs’ segment count is HIGHLY variable depending on the considered group Anal plate Genito-ventral plate Breathing through tracheae if present (openings usually directed to the front half of the body) Legs’ coxae can be fused to the idiosoma in some groups Caruncle/ Pre-tarsus Sternal plate
Some apulmonate arachnids: Opiliones (Harvestmen), Palpigradi, Solifugae and the mite body plan
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