A few nights ago, as I was walking towards my cabin along the edge of the Chitengo Camp, I heard a call of a cricket that I did not quite recognize. Cricket calls are unmistakable for their clean, almost melodious quality, very different from the call of a cicada or a katydid, which tend to be more “noisy.” But his call was very pure, almost bird-like, and it was coming from high in a tree. I expected to find a black, wide-winged Homoeogryllus cricket among the leaves, but to my delight the mystery caller turned out to be a gorgeous Blue-legged sylvan katydid (Zabalius ophthalmicus). This and a few related species produce some of the most pure tone, almost whistle-like calls, a rarity among katydids.
Sylvan katydids (Pseudophyllinae) are uncommon in this part of the continent. The greatest diversity of these insects is found in the rainforests of Central and West Africa, where most live high in the canopy. In Mozambique I expected to find only four species, and indeed found them all in Gorongosa (which of course is not to say that more are not to be discovered.) Virtually all sylvan katydids, true to their common name, are associated with forests and other woody habitats, and few are found in the open savanna.
Southern African sylvan katydids display two very distinct types of mimesis (camouflage). Species found in the canopy of broad-leaved trees (e.g., Ficus) are superb mimics of foliage, complete with leaf-like venation of their wings and fake fungal spots or other “damage.” During the day they rest with their wings spread flat against the lower surface of the leaf, and are absolutely impossible to find. (Interestingly, this type of behavior does not occur in any katydid species found in the New World – all North and South American katydids hold their wings in a vertical position, and during the day rest on twigs with their wings facing up in an imitation of a small leaf.)
The second group of species of Mozambican sylvan katydids are bark mimics. These species are associated with more open habitats, mostly miombo or mopane woodland, and spend the day resting on trunks of small-leaved trees, such as Acacia or Brachystegia. Their wings are held similarly flat against the bark, and their coloration is mottled, resembling the surface of the trunk. In addition, their legs are strongly flattened and covered with dense hairs, which helps them eliminate the shadow cast by their bodies.
One of the Mozambican species, The Common bark katydid (Cymatomera denticollis), is unusual among katydids in its ability to produce chemical defenses. Production of repellant chemicals has been documented in a few Neotropical species, but this is the first example of such a behavior in an African species. These insects, when threatened by a predator, fan their wings and reveal a brightly colored, red, orange, and black abdomen. At the same time a gland on their abdomen sprays a strongly smelling liquid. I have had no chance to look into the chemical composition of this substance, but its smell is very much reminiscent of that produced by their (unrelated) South American counterparts. In those katydids the repellant substances were identified as methylpyrazines, and I would not be surprised if the African species produced related compounds.