Milkweed Tussock Caterpillars on Common Milkweed
There is a common misconception in some quarters that anything “natural” is healthy and beneficial. Nothing could be further from the truth! The dominance of humans on planet earth is primarily linked to cultivation and domestication of a small number of plants and many of the rest are toxic. Indeed in some tropical environments humans only exist in primitive circumstances by figuring out how to remove the toxins from starchy plants (manioc and sago palm for example). It has even been suggested that the evolution of the human brain came about because early plant eating primates required a complex brain to remember what to eat when, and some plants countered with mind-altering chemicals that interfered with such thought. In any case I noticed many examples this week of the continued evolutionary arms race between predator and prey, and herbivores and their food plants.
In mid summer the pastures are often covered with an exotic but beautiful blue flower, chicory. Apparently cattle do not eat it so it thrives especially in pastures and on the sides of roads where it has less competition with grasses. A somewhat similar situation occurs with the spectacular but apparently toxic cardinal flower, which thrives only in disturbed water-side habitats or in pastures in wet meadows where competition with grasses is reduced by grazing by horses and cattle.
Many animals reduce their chances of being eaten by camouflage. The katydid is an astonishing mimic of green leaves; it even has leaf-vein like structures indicating how there has been selection not only for color but also for fine leaf-like structure. Similarly a prominent moth caterpillar I found on iron weed was a mimic of brown leaves and twigs. When discovered it reared up and assumed a somewhat threatening pose; it was also hard to tell the head from the tail. Clearly bird predators are very carefully examining plants for insect food and only the best mimics survive, and thus pass on the most adaptive traits to their progeny.
Some insects are toxic and advertise their presence and their distastefulness to potential predators. Everyone is familiar with monarchs and their brightly colored toxic caterpillars that derive their protection from chemicals in their food plant. Another of the milkweed-dependent group of insects is the milkweed tussock moth; the caterpillars are very brightly colored, as well as covered with hairs and bristles and presumably discourage most predators with their food-derived toxicity. Advertisement of toxicity is somewhat less common in vertebrates but I have been seeing red salamanders in our yard occasionally and they are famous for their bright red color which warns of toxic mucus, and in their mimic resemblance to the even more toxic red eft stage of the newt.
Probably the most interesting and surprising strategy used by some insects is the bluff or “scary eyes” tactic. We have many common buckeye butterflies and they fly slowly and often stop to bask. They are not protected by toxins to my knowledge yet how do they deter predators? Are birds really afraid of these tiny but distinctive eye marks? Or is the purpose of the eye spots to direct attacks to the spots rather than the vulnerable head and body of the butterfly? For example look at this buckeye which is quite worn and thus an older individual, yet it does not show large bird peck marks on the wings, just some fraying edges. So clearly buckeyes and various other insects that have eye spots show that some predators (especially birds) are quite alert to anything unusual about prey that might indicate poison or the ability to fight back. This has apparently led to the evolution of such detailed eye spots.
Bill Dunson, Englewood, FL & Galax, VA
wdunson@comcast.net