Giant Ichneumon wasp (Megaryssa macrurus, Linneaus 1771) ovipositing.
10th of August 2013, Georgia State Botanical Garden, Athens, GA. 3:47 pm.
This observation has been registered in iNaturalist.org with ID# 418639.
Oviposition marks the beginning of the life cycle of all insects including parasitoid wasps. Females of Megarryhssa macrurus (Hymenoptera: Ichneumonidae) are specialized to lay their eggs in the burrows of wood eating Pigeon Tremex Horntail (Tremex columba) larvae between June and September. Only one egg is deposited per host larva buried inside the wood. Eggs act as ectoparasitoids – a parasite that lives externally on another animal and eventually kills it – and are oviposited directly on the host larvae. Upon hatching, the larva begin feeding on the horntail larva throughout the winter. In the spring they pupate within the same burrow and undergo a complete metamorphosis. Between the months of June and September they emerge from their burrows as adults. Once the wasp emerges it lives for up to 27 days until it mates and oviposits.
The details of the oviposition is quite impressive and complex. First photographic documentation of the behavior was done in 1916 by Fyles. In 1999, Canadian researchers have articulated morphology of the ovipositor using electron microscopy. Inspired by the ovipositor of this wasp researchers from Imperial College London are developing a steerable needle technology for difficult medical operations. Biomimicry of the wasp ovipositor may help design of minimally invasive surgical devices for delicate tissues such as the brain.
Oviposition begins after a lengthy search period. The female wasp inspects the drilling location using her sensitive antennae (behavior shown in more detail in the video below). Once the spot is located she marks the position by standing about half centimeter ahead of it with her middle legs (P2 legs). Then she makes a very tall stance lifting her abdomen to bring the tip of the ovipositor (stylus) in the drilling spot and inserts it almost vertically into the wood. This is not an easy task. The tip of the ovipositor almost always gets stuck to tiny irregularities on the wood surface. She also can loose her balance very often and has to start from the beginning since the exact point of drilling is crucial and must be recalculated for accuracy.
At first glance, the ovipositor seems like a single filament. If you examine closer (without scaring her of course) you can notice that it actually has three filaments. The middle filament is the ovipositor which drills into wood. The other two side filaments are called valvulae (a cylinder-like sheath supporting ovipositor or sting) and guide the ovipositor into the drilling spot. Although very thin, ovipositor is a tube and the egg moves down the minute channel in its center during egg laying. Two other thin filaments (valvulae) serve as protection for the ovipositor. They arc out to the sides during egg laying. These parts have a cutting edge at the tip. They interlock and slide against each other. Even though the legs and abdomen are fully stretched they only serve to stabilize the extremely long ovipositor. Force required for piercing of the wood comes from a membrane between abdominal segments VII and VIII. A very stretchable membrane between those segments enables a 270 degree unfolding of the last abdominal segment. The illustration above shows the sequential steps in the abdominal segments (a through i). Beginning from step (f) the intersegmentary membrane forms a translucent bag and stretches as it pushes the stylus (f through g). Internal surface of this membrane is made of a single-cell thick layer of secretory epithelium tissue. A pear-shaped diverticulum (c) is probably responsible from secretion of a fluid that helps ovipositor to penetrate wood. Secretion must be containing lytic enzymes that digest wood. Analysis of the wood sections under scanning electron microscope after oviposition clearly shows traces of such enzymatic digestion activity. (Illustration from Le Lannic and Nenon 1999)
Each oviposition event can last more than 40 minutes making the female wasp vulnerable to many threats during that long period of time. Ovipositor is inserted fully and retracted about halfway back a number of times. Oviposition must be energetically very exhausting. Tired individuals can loose their grip and can even injure themselves by twisting the long ovipositor. Site of oviposition is a limited resource and can lead to aggressive competitive behavior among females of the same species attracted to the same spot. The following video shows harassment of an individual by another female attracted to the site. You can see the antennae interaction resembling a fencing competition.
If your coefficient of curiosity hit 1.00 you can read more about parasitic wasps in SciBlog post of Christopher Buddle.