Monday, January 13, 2014

Metal Mouth

D. irritator female  Tom Murray
The previous blog discussed ichneumon wasps that bore deep into trees to deposit their eggs on the larvae of bark-boring beetles.  Their long and delicate looking ovipositor can be over twice as long as their body, and curves dramatically as they drill into the tree.  Considering that they may drill through over two inches of wood to reach their victim, will do this for 10 separate eggs and may drill a dry hole, missing the larva, up to 90% of the time, their persistence is amazing.  Even more daunting is the wear and tear of that delicate looking ovipositor.  How do they keep from wearing it down to a dull nub?  One answer is heavy metal.

Long ovipositor-

In a study in 1998,  Donald L. J. Quicke and colleagues found that many genera of hymenoptera (sawflies, wasps, bees and ants)) incorporate metal in the cuticle of their ovipositors and jaws which reduces abrasive wear.  The ovipositors are adaptations of what we would call "stingers," used to position the egg on an unsuspecting larvae of beetles, lepidoptera (butterflies and moths) and even other hymenoptera.  The metal can make up to 10% of the ovipositor's weight. Many of these same species have metal deposition in the adult's jaws.  This is necessary to chew its way out of the host plant once it emerges from the pupa.  In general, no metal is found in the ovipositors of hymenoptera which penetrate soft materials or don't penetrate in egg laying.

"Stump stabber"-Adrian D. Thysse
Some species such as horntail wasps utilize zinc while our ichneumon wasps previously described and Cynipoidea or gall wasps incorporate manganese.  Gall wasps bore into plants to deposit eggs that subsequently produce the plant galls that shelter the hatching larva as well as providing its food.  New research in 2013 shows a direct correlation between the hardness of the substrate the gall wasp is penetrating and the amount of metal (zinc, magnesium and also copper) in their body parts.

Hymenoptera are not alone in their use of metal.  The teeth of some caterpillars have been shown to be hardened by zinc deposition.*  How do you determine metal without tedious drilling after asking the cat to "open wide?" Scanning electron micrograph images detect the presence and distribution of zinc in both the jaws and the tarsi of some forest caterpillars that chew into the hardened lignin of living trees.  Details of the 1999 study are at livingwithinsects.

Zinc deposition in caterpillar mandible *
There are dramatic serial pictures here of a giant ichneumon drilling which show the long curved ovipositor slowly disappearing into the tree trunk, its looping curve suggesting that there is no substantial pressure pushing it in.  You can watch the process on this 3 minute video.

*A. R. FONTAINE, N. OLSEN, R. A. RING & C. L. SINGLA. 1991. Cuticular metal hardening of mouthparts and claws of some forest insects of British Columbia. J. ENTOMOL. Soc. BRIT. COLUMBIA 88: 45-55.