Thread Snakes

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Family Leptotyphlopidae

Threadsnakes may be the oldest, or at least one of the oldest, snake lineages on the planet. Molecular clock data suggests they last shared an ancestor with all other living snakes about 155 MYA. Members of the threadsnake genus Rena have 14 midbody scale rows, 10 or 12 scale rows on the mid-tail, 168–312 middorsal scale rows, 9–21 subcaudals, 2–3 supralabials, moderate or large (rarely small) anterior supralabials. Adult reach lengths of 205–389 mm. A body shape of 26–60 (calculated by dividing the total length by the width). A relative tail length of 3.1–8.6 %. A tail shape of 1.9–3.8 (calculated by dividing the total length by the width). The patterns lack stripes. They have small supraocular scales. The genus Rena was established by Baird and Girard (1853) for two species R. dulcis and R. humilis.

Gehlbach and Baldridge (1987) reported Eastern Screech Owls capturing live thread snakes and bring them to their nests, while other prey was delivered dead to the chicks. The owls eat some of the snakes, but most snakes stayed alive in nest debris, where they fed upon soft-bodied insect larvae that were part of the decomposer community in fecal matter, pellets, and uneaten prey in the nest. The snakes may reduce parasites on nestlings or larval insect competition with nestlings for food cached in the nest. Nestling owls living with the thread snakes grew faster and experience lower mortality than same-season broods lacking snakes. The authors propose a commensal relationship in which the Screech Owl benefits reproductively and the live-in blind snake is not affected. Presumable other screech owls, and possibly other owls, use threadsnakes in a similar way. Threadsnakes and their scolecophidian relative the blind snakes, Typhlopidae are remarkably resilient. There is also an account (O’Shea et al. 2013) demonstrating scolecophidian snakes have morphological and physiological adaptations that allow them to survive passage through the digestive system of predators. O’Shea et al. (2013) found a live blind snake emerging from the cloacal opening of a toad, and the same behavior cannot be ruled out for birds. Thus, birds could be responsible for distributing these snakes a substantial distance from their home range.

Rena humilis Baird and Girard 1853

Western Threadsnake

This snake has also been called: Stenostoma humile — Peters 1858; Catadon dugesii Bocourt 1881; Siagonodon dugesii — Bocourt 1882; Glauconia humilis — Boulenger 1893; Leptotyphlops humilis — Ruthven 1907; Leptotyphlops humilis humilis — Klauber 1931; Rena humilis — Adalsteinsson, Branch, Trape, Vitt & Hedges 2009.

Rena humilis is based upon a specimen collected by John L. Le Conte at “Valliecitas,” California. This is now the city of Vallecito in San Diego County, according to Klauber (1931). The holotype was described by Baird and Girard in 1853.

For much of the 20th century this species formerly was included in the genus Leptotyphlops, however Adalsteinsson et al. (2009) examined DNA sequences from both both mitochondrial and nuclear genes of 91 individuals representing 34 species of the family Leptotyphlopidae.

They removed the North America members of the leptotyphlopids from the

genus Leptotyphlops and revalidated Baird and Girard’s genus Rena. The genus Rena was established by Baird and Girard (1853) for two species: R. dulcis and R. humilis. The name is feminine and derived from the Latin noun ren for kidney, likely a reference to the red brown kidney color of the type species.

Adults can reach 339 mm in total length. Hatchlings are about 90 mm in total length. The head is the same width as the body. The eyes are probably non-functional as producing an image in the snake’s brain, but probably act as light receptors. They are covered by a translucent ocular scale and appear only as a small dark spot under the scale. Scales are in 14 rows and are uniform in size around the body; they are glossy and hard.

This is a very thin snake with a blunt head and the tail is blunt and terminates in downward curved spine. The scales are shiny and cycloid. The ventral plates are slightly larger than the dorsal scales and look much like the dorsal scales. The lower jaw is countersunk. The snake can be brown, purple, or pink and may be mistaken for a worm.

Hahn (1977) reported R. h. humilis as having of 12 scale rows around the middle of the tail, more than 257 dorsals (257-283, x = 272), 7 to 9 pigmented dorsal most scale rows, fifth mid dorsal scale not much wider, if any, than sixth, and 15-21 (x = 17.9) subcaudals.

He diagnosed L. h. segregus as having a combination of 10 scale rows around the tail, more than 250 dorsals (261-275, x = 271), 12-16 subcaudals (x = 14), and the 7 dorsal most scale rows pigmented.

Based upon his map he considered the Santa Rita area population intergrades between R. h. humils and R. h. segregus. However, the Trans-Pecos threadsnake, Rena humilis segregus, occurs in southeastern Arizona, presumably east of the Cochise filter barrier and has 10 rows of scales around the tail, 261 to 275 dorsals, and 12 to 16 subcaudals. Wallach et al. (2014) considers R. h. segregus a distinct species – R. segrega. There are at least two specimens of R. segrega (not examined by me) from Pima County, Arizona (SDNHM 32783, 41801). Note the spelling changes so that the gender of the

genus and species agree.

Diagnosis: Rena humilis/segrera can be distinguished from all other snakes by the absence of supraocular scales, the presence of a prefrontal scale, and nasal scales which do not extend posterior to the eye. Thus, there may be two species if threadsnakes present in the Santa Rita area. Rena humils has 12 scale rows around the tail and R. segrega has 10 rows.

The Western Threadsnake inhabits desert scrub to interior chaparral biomes in the Lower and Upper Sonoran life-zones from sea level to about 3500 feet. They are usually associated with loose, moist soils and ants and termite activity (Kay 1970). The species occurs from the Big Bend area of west Texas westward to southern California, north to southern Nevada, southwestern Utah and south central New Mexico, and southward into Mexico in Colima and Baja California, including the islands of Santa Catalina, Carmen, Cerralvo and Cedros.

All the specimens we are aware of come from west of the Santa Rita’s at elevations below 3500 feet. Localities are often semi-urbanized areas where ants and termites are plentiful. Punzo (1974) found all 17 stomachs he examined contain food as well as small quantities of sand likely ingested with prey. Rena humilis feeds on a variety of arthropods in addition to ants (Hymenoptera) and termites (Isoptera), but these two insect orders comprised 54.2% of the total diet. In addition, the diet of Rena humilis is also characterized by a greater percentage of fast-moving, surface dwellers (Arachnida: 7.5 per cent; Orthoptera: 9.6 per cent; Diplopoda: 42.9%; Phalangida: 64.3 %) suggesting that Rena humilis is actively foraging on the surface. The predominance of larvae and nymph prey-types as compared adult insects indicates a preference for soft-bodied arthropods. Most adult prey consumed were spiders and Orthoptera which are soft-bodied. Punzo’s 64.3 per cent) suggesting that Rena humilis is actively foraging on the surface. The predominance of larvae and nymph prey-types as compared adult insects indicates a preference for soft-bodied arthropods. Most adult prey consumed were spiders and Orthoptera which are soft-bodied. Punzo’s data also indicates that several insect species which live as commensals in ant and termite nests such as lycaenids (Lepidoptera), phorids (Diptera) and clavigerids (Coleoptera), are readily consumed along with their hosts. Thus R. humilis is opportunistic and consume most soft-bodied arthropods that are encountered. He also found high percentage of polyphagids (Coleoptera), beetles that are found almost exclusively within decaying clumps of cacti, a microhabitat that is frequently visited by foraged snakes. It should be noted that all Punzo specimens came from Cochise County and may refer to Rena segrega.

Reproduction is poorly known, in southern California spermatogenesis starts in the early spring, most females are carrying eggs in early summer, and are probably laid in late summer. The smallest female with eggs was 248 mm with a clutch size of three. There is likely a long list of possible predators of this snake, but an unexpected predator is the Variegated Gecko. Babb and Brennan (2013) observed a Variegated Gecko, Coleonyx variegata unsuccessfully attempt to prey on a threadsnake.

John Lawrence LeConte graduated from medical college and traveled in California and Panama in 1849. He was a collector and naturalist and sent 10,000 beetles preserved in alcohol back to his father. He spent two years exploring the Colorado River, and was in Honduras for the building of the Honduras Interoceanic Railway, and in Colorado and New Mexico with the party surveying for the Kansas Pacific Railroad. He moved to Philadelphia in 1852 and resided there for the remainder of his life. LeConte was active in scientific societies including the American Philosophical Society and the American Association for the Advancement of Science. He was a founder of the American Entomological Society, and a charter member of the National Academy of Sciences. The snake genus Contia and several hundred species are named after him. LeConte collected birds and other natural history specimens for Spencer Fullerton Baird, a distant cousin and director of the Smithsonian Institution for a total of 39 years. In turn Baird asked other naturalists to collect beetles for LeConte. The holotype of Rena humilis was collected by LeConte.