That is, they don’t leave their leaves…
A recent study* tested the influence of leaf age on choice in four species of phasmids from Malaysia, two specialist species (i.e. choosy insects feeding on a few specific plants), and two generalists (feeding on a broad spectrum of plant hosts).
The study was motivated by the amazing fact that generalist species tend to prefer older leaves, while specialists are keen on young leaves. It is rather intriguing, given that young leaves usually contain many noxious substances, while the old leaves are less toxic. Indeed, a young leaf is costly and is of great value to a plant, and the latter doesn’t really “want” it to be eaten straight away, while older leaves are more costly to maintain. But the pattern of phasmid herbivory is logical at least: you expect specialist species to have evolved resistance or tolerance to their host chemical defences (even probably prior to the feeding specialisation), and younger leaves are more nutritive. On the other hand, generalist herbivores probably cannot maintain a costly tolerance, given their broad diet (i.e. a plant-specific tolerance is of lesser interest if the plant isn’t a major part of the diet, and insects would have to evolve multiple tolerance/resistance pathways, one for each plant host).
Surprise, among the studied species, Lonchodes cultratolobatus (also known as Lonchodes hosei in European stick insect strains –phasmid entomologists just seem to have a love for synonymy ;), a species I reared a few years ago… So there’s a picture (a female adult), just above. Note that the head is toward the left, not on the right.
The experiment tried to identify chemicals that would enhance or deter a feeding response in the different phasmid species, and leaves were sprayed different extracts from either young or old leaves. Unfortunately, a single component doesn’t seem to lead to a feeding response in stick insects, but extracts usually trigger this behaviour in expected ways (generalist species prefer old leaves/ specialists forage on younger leaves). This means leaf chemicals probably do not elicit behaviours alone, but a mixture would. Finally, leaves compounds can either act as deterrent or stimulant.
One more interesting observation is that generalist phasmids let their eggs fall apart, and don’t really care. On the other hand, specialist species usually stick their eggs onto the plants they eat. This is than convenient to newborn insects not to have to look for their food too much… Particularly when their diet consists of a single plant species…
* Bluthgen, N. and A. Metzner (2007). Contrasting leaf age preferences of specialist and generalist stick insects (Phasmida). Oikos 116: 1853-1862.