Some characteristics of Onychophora

Euperipatoides rowelli

Principapillatus hitoyensis ejecting its glue-like slime (Photo: Alexander Baer)

One of the most peculiar features of velvet worms is their use of a sticky slime secretion for defence and prey capture, which is ejected via a pair of modified limbs called slime papillae (see the image above) [1]. The slime is produced and stored in large glands situated on each side of the gut within the body [2]. The ejected threads of the glue-like slime entangle the prey (crickets, amphipods, wood lice and other small invertebrates), after which the prey’s cuticle is punctured with a pair of jaws and its body is injected with digestive saliva [2, 3]. The liquefied contents are then ingested using a sucking pharynx [4].

The scientific name Onychophora means "claw-bearers" (Greek: όνυξ = nail/claw and φέρω = carry/bear) and refers to a pair of sclerotized claws on the tip of each leg [5], while the English name "velvet worms" highlights the velvety appearance of the water-repellent onychophoran body surface [6]. The chitinous cuticle is moulted periodically, mediated by ecdysteroid hormones [7]. Hence, onychophorans are typical ecdysozoans. Within the Ecdysozoa, Onychophora is united with Tardigrada and Arthropoda in the clade Panarthropoda [8], but the exact relationship of these three panarthropod groups is discussed controversially [9].

The overall anatomy of onychophorans has remained largely unchanged in the last 540 million years, as they resemble the Early Cambrian lobopodians, such as ✝Aysheaia pedunculata from the Burgess Shale formation in Canada and ✝Onychodictyon ferox from the Chengjiang fauna of China [10]. The extant onychophorans share with the extinct lobopodians unjointed limbs (=lobopods), a soft body without an exoskeleton, and homonomous segmentation [11]. Due to these ancestral features, onychophorans are an important outgroup for understanding the evolutionary changes that have taken place in the arthropod lineage.


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