Physical characteristics

The family Leptotyphlopidae includes the most highly miniaturized snakes in the world. Although a few species (e.g., Leptotyphlops humilis, L. melanotermus, L. occidentals, L. tricolor, L. weyrauchi, and Rhinoleptus koniagui) occasionally grow to lengths of over 1 ft (30 cm), most forms are significantly smaller, ranging between 4 and 10 in (10 and 25 cm) in total length and often weighing less than 0.05 oz (1.4 g). Even more remarkable than their short length, however, is their ex tremely narrow build, a characteristic reflected in their common names, "slender blindsnakes," "threadsnakes," and "wormsnakes." Most species attain a maximum body width of only 0.04-0.20 in (0.1-0.5 cm) and exhibit aspect ratios (total length divided by body width) of between 40 and 100. Two exceptionally slender species, L. macrorhynchus and L. occidentals, occasionally have aspect ratios exceeding 140, and even the stoutest forms (e.g., L. broadleyi and L. boulengeri) are more slender than most other snakes, rarely having aspect ratios of less than 30.

Leptotyphlopids bear a strong superficial resemblance to other blindsnakes (Anomalepididae and Typhlopidae) in having cylindrical bodies covered by smooth, equally sized, cycloid scales, short lower jaws countersunk into the ventral surface of the head, and vestigial eyes that are scarcely visible beneath the enlarged head scales. (However, in some species, most notably Leptotyphlops macrops, the eyes are larger and more highly developed.) In addition, all forms have numerous tactile organs housed within the anterior head scales, often visible to the naked eye as tiny, light-colored specks on the external surfaces of the scales. However, several morphological characteristics serve to distinguish leptotyphlopids from anomalepidid and typhlopid blindsnakes. In particular, all slender blindsnakes have either 14 (in Leptotyphlops) or 16 (in Rhinoleptus) rows of scales encircling the body (all anomale-pidids and nearly all typhlopids have more than 16 scale rows), a single anal shield (all anomalepidids and nearly all typhlopids have two or more), and a distinctive arrangement of the scales along the upper lip. Moreover, leptotyphlopids are unique among snakes in having teeth only on the lower jaw.

Slender blindsnakes are generally rather dull in appearance. Although a few South American species (e.g., Leptotyphlops

World Smallest Bill
The black threadsnake (Leptotyphlops nigricans), one of the world's smallest snakes, with rice grain-size eggs that are strung together like sausages. (Photo by Bill Branch. Reproduced by permission.)

alfredschmidti, L. teaguei, L. tricolor) are boldly patterned with multicolored dorsal stripes, most leptotyphlopids are pat-ternless and have a relatively uniform pink, gray, tan, brown, or black dorsal coloration. Those forms that are pinkish in color, such as the two species found in the southwestern United States (L. dulcis and L. humilis), bear an uncanny superficial resemblance to earthworms, thus giving rise to another common name for these diminutive serpents, "wormsnakes."

The size and shape of both the snout and tail are somewhat variable within Leptotyphlopidae. Most species of Leptotyphlops have relatively blunt, rounded snouts. However, several Old World species (e.g., L. macrorhynchus, L. parkeri, L. rostratus) have prominent, hook-shaped snouts, and in two Socotran forms (L. filiformis and L. macrurus), the snout is both hooked and pointed. A pointed snout is also seen in Rhi-noleptus koniagui. Such highly derived snout morphologies are less common among New World taxa, but they are seen in a few South American species (e.g., L. borrichianus and L. un-guirostris). As in most other blindsnakes, the scales surrounding the snout in leptotyphlopids are somewhat larger than those surrounding the body, and in at least one species (L. humilis), the largest of these scales (the rostral) fluoresces under ultraviolet light. In most taxa, the tail constitutes 5-10% of the snake's total length, but this figure may be as low as 2.1% in short-tailed species (e.g., L. septemstriatus), or as high as 18.9% in long-tailed species (e.g., L. macrurus and L. wilsoni). The tail usually terminates in a small needle- or thorn-shaped apical spine.

Leptotyphlopids are also characterized by a number of distinctive internal anatomical features. The most significant of these relate to the structure of the jaws. The upper jaws are toothless and relatively immobile. In contrast, the lower jaw bears teeth and is highly flexible due to the presence of exceptionally well-developed intramandibular joints, which divide the left and right halves of the lower jaw into separate anterior and posterior segments. Also unusual are the form and position of the hyoid apparatus, which is Y-shaped and located far behind the head (characteristics also seen in ty-phlopid blindsnakes). The pelvic apparatus is, in general, more complete than that of other snakes, typically consisting of paired ilia, ischia, pubes, and femora (although in some taxa the pelvis is highly reduced [e.g., Leptotyphlops macrorhynchus] or absent [e.g., L. cairi]). Even in species that possess well-developed femora, however, the horny spurs on the distal ends of the femora rarely protrude through the skin as they commonly do in other basal snakes (e.g., pipesnakes, boas, pythons). Perhaps the most bizarre osteological feature of Leptotyphlopidae is seen in several Old World species of Lep-totyphlops (e.g., L. cairi, L. macrorhynchus, L. nursii, and L. oc-cidentalis), in which much of the skull roof has been lost.

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