Readers’ wildlife photos

Today we have photos of stick-mimicking insects from Trinidad and Tobago, all taken by Ephraim Heller. Ephraim’s captions and IDs are indented, and you can enlarge the photos by clicking on them.

I remember the fascination I felt as a child on the rare occasions when I was taken to a zoo that had a terrarium containing stick insects. I still feel that way. In researching this post, I discovered that stick insects are even more remarkable and unusual than I anticipated. For example, parthenogenesis is common; they regrow lost limbs; and the world’s longest insect is Phryganistria chinensis, found in China and measuring 36 cm in body length (62 cm or 2 feet with legs extended, photo here).

I photographed two species. The first four photos are the Trinidad log insect (Phanocles keratosqueleton), known in regional folklore as the “god horse” or “hag’s horse.” It appears in folklore as an omen of death, despite being a harmless herbivore.

I never found a stick insect during our daytime hikes. During daytime, stick insects press themselves flat against plants and remain motionless, rendering them camouflaged and invisible. After dark, they walk out onto exposed vegetation to feed, molt, and mate. They are easily spotted with a headlamp due to their eyeshine:

Their camouflage can incorporate three distinct adaptations: cryptic coloration and background matching; cryptic body shape and texture; and behavioral crypsis (swaying when disturbed, mimicking a twig moving in a breeze). Not only are the insects themselves camouflaged, but many species evolved eggs that look like plant seeds:

Stick insects are in the order Phasmatodea, which contains over 3,500 species. Phasmids sits under Polyneoptera, which contains other winged insects such as grasshoppers, mantises, stoneflies, and earwigs. They are found on all continents except Antarctica. Against my expectations, Phasmatodea is monophyletic: the group evolved once from a single common ancestor, rather than through convergent evolution:

The next six photos are of the Trinidad twig or Trinidad stick (Ocnophiloidea regularis). More details on this species are at the end:

The oldest phasmid fossil is about 165 million years old, but recent studies claim that Phasmatodea first evolved 252 – 299 million years ago. This suggests that they evolved in response to the radiation of early insectivorous vertebrates such as parareptiles, amphibians, and synapsids. A major diversification occurred in the late Cretaceous, with the rapid spread of flowering plants (providing new foliage types to mimic) and the emergence of early birds:

Stick insect species’ reproduction ranges from sexual to obligate parthenogenesis, and much in between. Parthenogenesis (reproduction without fertilization) is common and has evolved independently many times among phasmids. Parthenogenic offspring are almost always females, producing all-female or near-all-female lineages. The offspring are not true clones of the parent, but are typically homozygous and have reduced genetic diversity, which can impair their ability to adapt to new stresses. Some species are facultatively parthenogenetic, meaning females can switch between sexual and asexual reproduction depending on conditions:

Phasmids can voluntarily shed a leg when grabbed by a predator. The leg is broken off at a specialized weak joint. Phasmatodea is the only insect order known to regenerate lost legs. Regeneration is restricted to nymphs because it requires molting. Cells at the wound site dedifferentiate and form a mass called a blastema, which then rebuilds the limb segment by segment through successive molts. The same molecular signaling pathway (ERK/CK2) involved in vertebrate limb regeneration drives the process in stick insects, which has attracted research interest for regenerative medicine. Regeneration is not free. Regrowing a leg during development results in disproportionately smaller wings and measurably reduced flight performance in adults. The body appears to divert resources away from wing development to fund limb repair:

The Trinidad twig (photos above and below) reproduces sexually:

The photo below shows two males attached to a female. Phasmids don’t do polycules and this is not standard reproductive behavior, but research on a closely related species has documented this scenario. While one male is guarding a female by remaining clasped to her abdomen, a rival male can approach and attempt to insert his genitalia while the first mate is momentarily repositioning or feeding. If the rival succeeds in attaching, both males end up simultaneously clasped to the female. This can result in a slow-motion “boxing-like” confrontation, with both males leaning backward and suspended from the female while trading blows with their forelegs until one of the males is eventually displaced: