Thursday, January 22, 2015

The case of the hunting hymenoptera

Jean-Henri Fabre
In 1879, the French entomologist J. H. Fabre studied many species of hymenoptera (solitary wasps and bees) that hunt other insects as food for their larvae. This is the reason for their name (hunting hymenoptera, also called -improperly- parasite hymenoptera). Before laying the eggs, the hymenoptera paralyze the prey by injecting with their sting a drop of poison in every nerve ganglion in the un-centralized nervous system of the prey. In some species, such as Ammophila hirsuta, which hunts caterpillars, the number of ganglions may be large (up to twelve, one per segment in the caterpillar). The hunter seems to know where exactly its prey must be stabbed with the sting.
Once the prey has been paralyzed and the egg laid, the minute larva of the hymenopter digs inside the prey and starts devouring it, showing an apparent innate knowledge of the prey anatomy: it starts feeding on the parts less necessary for life, leaving the vital organs to the last. In this way, the prey does not die and rot, which would make it improper as food and lead to the death of the predator.
Ammophila sabulosa carrying a hunted caterpillar
Fabre described in detail the behavior of Ammophila hirsuta in the first volume of his Souvenirs entomologiques and used it as an anti-evolutionary argument in his polite and rational controversy with Charles Darwin, which lasted until Darwin’s death, although in subsequent volumes of his book Fabre kept using the same argument, which can be summarized thus:
Natural selection and gradual evolution cannot be reconciled with the behavior of Ammophila hirsuta. A hypothetical missing link that would inject its poison in -let’s say- one half of the caterpillar segments would not be viable, as the prey would not be perfectly paralyzed and would kill the larva inside it with its upheavals and contortions. This behavior, therefore, must have appeared suddenly, or been directly created by God.
Henri Bergson
The argument was later used by the French philosopher Henri Bergson in his work L’évolution créatrice as an argument for his élan vital, his way of explaining life in the frame of his philosophical hypothesis on the predominance of spirit against matter. The argument could also be used as an example of irreducible complexity like those proposed by the advocates of intelligent design.
It is surprising, therefore, that Fabre himself had described what could be taken as a missing link for hunting hymenoptera such as Ammophila. In chapter 5 in the second volume of his Souvenirs he describes another genre of hymenoptera (Eumenes) whose larva, which feeds on partially paralyzed prey, hangs from the roof of the excavation by means of a thread that can be lengthened or shortened at will. This allows the larva to feed when its prey are motionless, and retire to safety when it could be harmed by their movements.
It is interesting that the case of the hunting hymenoptera has also been used by atheistic thinkers as an argument against design and the existence of God, by presenting it as a specially cruel example of the problem of evil or, in Tennison’s words, Nature, red in tooth and claw. Their version can be summarized thus:
The behavior of the hunting hymenoptera causes in their prey an indescribable pain. Therefore it cannot have been designed or wanted by a good God.
There are two different answers to this argument:
  1. In general, by saying with Saint Augustine (who offered this explanation in the IV century) that God permits evil because this is an essential condition to reach a greater good. See also C.S.Lewis on The problem of pain.
  2. The argument based on the assumed pain of the paralyzed caterpillars while they are being devoured is actually a case of anthropomorphism. We know almost nothing about the sense of pain in insects. It is well-known that cockroaches accidentally wounded may end-up eating their own leg (autophagy or self-cannibalism), which means that the sense of pain in insects is not like ours (just imagine yourself eating your own wounded arm). Therefore caterpillars being devoured by the larva of a hymenopter perhaps don’t feel so much pain, which weakens the argument. When we think about a paralyzed, slowly devoured prey, we tend to imagine what we would feel in such a situation. The moral suffering would probably be the worst, but it is likely that insects never feel that.
The same post in Spanish
Thematic thread on Evolution: Next

Manuel Alfonseca

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