Natatory fringes are rows of stiff hairs that occur along the margins of the hindfeet in some rodents. They occur along the plantar margins and in some cases also between the toes.Weksler, 2006, pp. 24–25 Among sigmodontines, a mostly South American groups, natatory fringes are present in Ichthyomyini and some Oryzomyini.Weksler, 2006, p.
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The pharynx usually shows no papillae around its opening, but they are present in larger species. Its proventricle is long and wide, barrel-shaped, with thin muscle cell rows, counting with around 15 to 22. Mature males exhibit natatory chaetae, while females brood eggs dorsally by means of capillary notochaetae.
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25 Among ichthyomyines, the fringes are poorly developed in Neusticomys but well-developed in other genera, and in Rheomys mexicanus the hairs of the fringes may exceed in length.Voss, 1988, p. 279 Amphinectomys, Holochilus, Lundomys, and Nectomys are the only oryzomyines with natatory fringes, but have them only weakly developed;Voss, 1988, p.
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The middle, and more especially the hind legs are adapted for swimming (natatory): they are greatly flattened and fringed with bristles that fold to aid swimming action. In contrast the front legs are long and adapted for grasping food or prey. In males the front tarsi have suckers, which are used to hold onto the slippery female during mating.
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Nectonematids also possess a blindly-ending intestine and double rows of dorsal and ventral cuticular natatory bristles. In males, sperm sacs attached to the dorsal epidermis are the gonads, while females possess a vesicle-rich tissue called a gono-parenchyne during early developmental stages. Additionally, spines are formed on nectonematid eggs after they make contact with seawater. Like all horsehair worms, there is a lack of excretory organs or blood.
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It is probable that the natatory lamellae are what makes such implausible leaps possible. The plates also may aid jumping on land, which Tridactylidae certainly can do impressively. The posterior tibiae also bear articulated spines near their tips, plus spurs longer than the hind tarsi, which may be entirely absent or else are at best vestigial, having only a single segment. The insect uses its hind tibial spurs for digging, which is unusual for an insect's hind leg.
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The species' body is small, its holotype possesses natatory chaetae, with a total length of and width of , including about 27 chaetigers. Its prostomium is ovate, showing 4 thick eyes in a trapezoidal arrangement, as well as 2 eyespots. Its antennae are spindle-shaped, its median antenna longer than its lateral antennae. The palps are similar in length to the prostomium or somewhat shorter, dorsally fused by a membrane, and containing a small distal notch, sometimes folded.
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Correspondingly, in support of their powerful jumping capabilities, the hind tibiae bear movable plates towards their distal ends. These vary in number according to the genus, and they are called natatory lamellae (meaning literally "swimming plates"). Ordinarily the insect keeps the plates closely pressed against its tibiae but it can fan them out for swimming, which most species can do very well, some even being able to dive and swim under water. Apart from scrabbling over the water or swimming, some species actually can jump off the water surface.
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Its great preservation has also allowed the examination of the internal muscular structure of the swimming leg. The coxal muscles were elongated towards an anterolateral orientation. This suggests the legs were maintained in a posterolateral position to walk in order, thus allowing sufficient stability at the animal's center of gravity, which was probably located near the second tergite (upper half of the segment). The presence of proximal podomeres and a terminal spine in the swimming leg indicates their primitive condition as natatory organs and their functional adaptation to walk.
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The Transverse Ligament of the Palmar Aponeurosis (TLPA) is a thin band of transverse fibers of the distal portion of the palmar aponeurosis. It runs deep and transverse to the longitudinally oriented pretendinous bands of the palmar fascial complex, and serves as an attachment point for the septa of Legueu and Juvara. The TLPA is also known as the "Ligament of Skoog". It should not be confused with the Natatory Ligament (also known as the "Superficial Transverse Metacarpal Ligament"), which runs parallel and distal to the TLPA, forming the webbing in between the bases of the fingers.
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The species' body is small, with a total length of and width of , including natatory chaetae from chaetiger 8 to 23 (from a total of 28). Its prostomium is oval, a bit wider than its length, and is contracted on its anterior segments; it shows 4 large eyes in a trapezoidal arrangement and 2 anterior eyespots. All of its antennae are similar, being mamilliform, its median antenna inserted between its posterior eyes and its lateral antenna in front of its anterior eyes. The palps are short, fused along their entire length except for a terminal notch, which possesses papillae.
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