The unique digestive ability of the Neodiprion (commonly called sawfly) could not have evolved step by step.
On the surface, few people would suspect that one small insect could have an unsettling effect on the basic tenets of the theory of evolution. But the larva or the common sawfly does just that. The sawfly larva, a rather ordinary looking creature as caterpillars go, has managed to carve out an ecological niche on the fragrant needles of the pine tree. At first glance this may not seem like a particularly noteworthy accomplishment. But in the small world of in sects it represents a radical departure from accepted environmental norms. To many small insects the pine tree is a virtual no-man's land. Its resins and oils, which may smell sweet to us humans, are highly toxic substances as far as the six leggers are concerned. But not to the sawfly larva! This little critter not only can tolerate pine oil and resins, but it even "knows how" to chew them up with-out being poisoned. While munching on its staple diet of pine needles, the larva somehow separates the poisonous oils and resins from the digestible pulp and stores them in two goiter-like sacks located at the sides of its oral cavity. If provoked, it instantly turns its
Sawfly Larva at Work: These wriggly little caterpillars inhabit the forests of Europe and North America and are notorious for defoliating various types of conifers and pines. The sawfly is aided in its curious dietary habits by a unique digestive system which allows it to regurgitate poisonous oils and resins found in these leaves. The yellow blobs on the larva at the left represent the unpalatable remains of its recent meal of eucalyptus leaves. - See PDF for picture
head toward its aggressor and secretes a drop of its stored liquid. This foul smelling brew is usually enough to discourage would-be predators such as spiders, ants, and birds. This neat trick could be compared to a person who eats a sandwich containing poisoned sausage, but who swallows just the bread while storing the sausage in a cheek pouch. But just how does the sawfly larva accomplish this phenomenal feat?
Forestalling a Major Case of Indigestion
First of all, for the sawfly to do this, the edible parts must somehow be separated from the inedible parts: secondly, each must go the right direction so that the resins and oils will not get into the digestive tract: and thirdly, the tissue of its storage sacks must be insensitive to the resin acids. The larva of the sawfly is able to perform these, functions quite well. Its storage sacks are covered with a chitinous membrane and are thus effectively protected. The muscle tissue of the sacks is so extraordinarily strong that one sawfly expert believes it helps in the separation of the digestible and indigestible ingredients. Exactly how the separation takes place - how the larva is able to let all resinous bits disappear in the sacks during the chewing process
ADULT CONIFER SAWFLY, or Neodiprion, is a serious pest of coniferous trees. A medium-sized insect, it is common throughout most of North America, except in the Midwest. - See PDF for picture
and eat only edible parts - is still a puzzle.
Questions Evolution Can't Answer
The sawfly's unique digestive system is more than just an oddity. It presents a number of thorny questions as far as the theory of evolution is concerned. First of all, try to imagine the difficulties some ancient variety of sawfly larva would have encountered had it tried to switch from a normal leafy diet to one of pine needles. According to evolutionary theory, the changes necessary to enable it to digest the pine needles could only have occurred through small mutations in a step-by-step fashion. But in the case of the sawfly larva, such piecemeal transitions become totally impractical, and one is immediately confronted with one of evolution's basic problems of logic. The sawfly larva could not live on pine needles until a completely developed mechanism which separated wholesome food from poisonous pine resin was working with perfect reliability. But such a mechanism, if it came gradually into existence by small mutations, would have developed only if the food already consisted of pine needles. Yet all the sawfly's intermediate transition forms (which evolution requires) would clearly have been unsatisfactory because they would have been unable to adequately cope with the poisonous effects of the pine needles. In that case the sawfly's evolutionary progress would have been brought to a standstill. On the other hand if there were no pine needles around for the larva to feed on, there would also have been no stimulus for it to develop any mechanism for digestive separation and protection. No matter how intelligibly the theory of evolution may present the reasons for the step-by-step development of already existing organs or parts of the body, the difficulties in trying to explain how certain independent anatomical-physiological systems could originate by evolution seem insurmountable. Science is still far from understanding how different organisms came into being, even though it sometimes seems that we understand "in principle."