Parasitoid wasps 3: drilling with ovipositor that is 3 to 4 times its body length.

The parasitoid wasp Megarhyssa macrurus is used to illustrate how the extra-long ovipositor is being handled for drilling into hard wood.

Photo 1. **© matthew_wills,** **some rights reserved (CC-BY-NC)**

Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

https://inaturalist.nz/observations/93025328

Photo 2. **© jdsommer,** **some rights reserved (CC-BY-NC)**

Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

https://inaturalist.nz/people/jdsommer

Photo 3. **© Mike Farley,** **some rights reserved (CC-BY-NC)**

Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

https://inaturalist.nz/observations/6021633

Giant Ichneumon Wasp (Megarhyssa macrurus) Ovipositing

Body length: up to 5 cm

Ovipositor length: up to 13 cm ( about 3 times its body length)

From the above pictures and videos we can see how the wasp deploys its extra-long ovipositor. It does so by curling a portion of its ovipositor into a disc-shaped bag, like the bag covering a badminton racket. This bag is double layered: the ventral cuticle of the 7^th abdominal segment forms the inner layer and the dorsal cuticle between the 7^th and 8^th abdominal segment forms the outer layer. Thus we can appreciate that although the ovipositor looks like it is inside the wasp’s abdomen; it is actually completely on the outside.

It has been observed that the size of the disc can be up to 2 cm in diameter (or ~6 cm in circumference). Imagine an elastic-like rubber sheet measuring 1 mm or less being stretched to 30 mm. This is super elastic. Watch the video above. Scientists believe this elasticity is due to the presence of the protein Resilin in the cuticle. Resilin is currently the most efficient, elastic protein known (Elvin et al., 2005). It enables fleas to jump 38 times its length.

The membranous disc bag is deployed by pushing the proximal hook end of ovipositor into the abdominal membranes, resulting in the proximal ovipositor being forced into the circumference of the expanding membranous disc. This is achieved by erecting the long abdomen (90° with respect to the surface of the tree trunk), bending the tip of the tail inward, and erecting ovipositor to drilling position. See fig. 1 below to understand the anatomy of the ovipositor complex.

Figures 1 and 2 depict the structures that make up the ovipositor complex. The complex looks like a sports shoe in side view. The anterior end has the bulb end of the 2^nd vulvulae (ball and socket joint), where the ovipositor is erected to the drilling position.

The ovipositor is now raised up to an acute angle, with the tip inserted into the tree trunk. The wasp then uses its erected, curved abdomen to apply force along the axis of the ovipositor. Since the tip of the ovipositor is inserted into a hard and immovable tree trunk, the other end is forced to curl up against the under belly of the wasp. Further pressure forces the ovipositor into the median abdominal groove. Additional pressure causes the whole ventral abdominal wall to bend inwards, resulting in a portion of the upward curving ovipositor to stay inside the confines of the “n” – shaped dorsal tergites, especially the 5^th and 6^th abdominal segments. This gives more support and stability to the long ovipositor.

Further up, the continuing rising and curving ovipositor forces the potential space between the 7^th and 8^th abdominal segment open. The two segments open up like a flower, to more than 180°, hinged only at the ventral margin. The inter-segmental membrane is stretched to a disc of 2 cm in diameter. While all this is going on, the ovipositor complex is also forced upward into the developing disc space, by turning about 80°. The end result of this sequence of events is astonishing. A large transparent disc is created at the sub-terminal end of the wasp containing the ovipositor complex, together with the initial portion of the ovipositor tube proper and its covering sheaths. However, the ovipositor complex is now turned upside down (the ventral side is now facing upwards, dorsally) and front to back (the tail end is now pointing towards the head end). Most of the above descriptions can be observed in the above Youtube video, best viewed under 0.25X normal speed, all within the 1st one minute.

As the ovipositor gets coiled up in the disc, the practical length gets shorter, and the wasp moves the tip, in steps, closer to its body, till it is nearly vertical. Actual drilling then begins.

The actual drilling process probably involves the so-called push-pull mechanism (King and Vincent, 1995), detailed in the previous article. During drilling, the left and the right 1st valvulae slide back and forth in turn at high speeds. While moving upwards, the left 1^st vulvula’s more proximal and larger upward pointing saw teeth will gouge out the wood on the side of the tunnel. This resistance to the upward motion of the left 1^st vulvula will act as a counter force to the downward motion of the right 1^st vulvula. This cyclic motion, whereby one valve is pulled backwards and anchored, whilst the other is pushed further into the substrate, enables the forward motion to be achieved with a near-zero net force. The chance of buckling is thus minimized.

The drill tip is also hardened by means of Sclerotization and zinc deposits.

While drilling is on-going, the wasp is constantly analysing the intercepted acoustic wood-chewing sounds made by the host larva. These sound vibrations are detected through the wood by the built-in acoustic receivers located in the wasp’s legs. There are also location sensors on the tip of the drill. This setup enables the wasp to constantly change the drill direction, to maneuver towards the location of the host larva.

In fact, scientists have done experiments to show that the wasps can drill in straight lines and are able to divert to any other direction, so as to bypass obstacles to reach the host larva. The research article below contains three movies/videos recordings of the actual bending paths of the ovipositor.

Mechanisms of ovipositor “insertion and steering” of a parasitic wasp

https://doi.org/10.1073/pnas.1706162114 Experimental Zoology Group, Wageningen University and Research, 6708WD Wageningen, The Netherlands

Watch movies1,s2 and s3 in the above scientific article – recordings of straight and curved paths of wasp ovipositor drillings.

Once the host larva tunnel is breached, the drill tip is rapidly converted to a hypodermic needle. A quick jab is given to the host larva to release the paralysing venom. The slender wasp egg is deposited outside the host larva. Ovipositor is then rapidly pulled up. All these steps have to be done in split seconds because if the larva is given a chance to struggle, the wasp’s ovipositor tip may be damaged. If that happens, it will be the end of the wasp.

We will now look at the above happenings in more details.

Conversion to hypodermic needle. The two ventral valves of the 1^st vulvulae are protracted a little bit beyond the tip of the dorsal valve to become the needle. However the portion of the 1^st valvulae that protrude out is now lacking a dorsal roof, potentially allowing venom to leak out. To re-create the missing roof, both ventral valves carry a dorsal chitin plate along each of their dorsal-medial edge. When both longitudinal plates meet in the middle, a new roof over the egg canal is created. Both dorsal plates are locked together by a mechanism similar to the olistheter mechanism.

There is another bigger potential leak from the egg canal along the whole length of the terebra, on the ventral side. This potential gap is between the two medial sides of the left and right valves of the 1^st valvulae. To plug up this potential leak, a longitudinal thin flap of elastic chitin grows out of the ventral-medial aspect of each of the two ventral valves, pointing towards the centre of the egg canal. When the wasp contracts its abdominal poison glands, there is a sudden increase in volume of venom within the egg canal. This sudden increase in pressure within the egg canal forces the two longitudinal flaps together and outwards. This action effectively closes off the ventral gap immediately. Thus, the venom goes directly through the water-tight egg canal into the larva to paralyse it.

The next task is to release the wasp egg outside the larva but within the larva tunnel.

The egg is kept in a holding area, a short distance from the tip. This section of the egg canal is larger and spindle-shaped. However, the external diameter of the terebra here is not increased. Hence, the larger egg canal size comes at the expense of a thinner dorsal and ventral valvulae. Not only that, but the ventral 1st valvulae, at this level, now takes up to 80% of the egg canal. The egg is prevented from moving further downward to the tip, by two transverse chitin plates growing out, one from each of the two ventral valves, near the distal end of the spindle-shaped egg cavity. The transverse plates are called valvilli and together they cause narrowing of the egg canal, effectively preventing the downward passage of the egg. In addition, these two valvilli grow upwards into the spindle-shaped, enlarged egg cavity. They are concave in shape, akin to a pair of hands stretched upwards to catch the downward moving wasp egg, and enclose it in a protective cocoon. All the above anatomical modifications help to ensure a rapid, safe and accurate release of the egg.

Now, with the anatomy explained, we go back to the action.

The 1st valvulae is protruded like a hypodermic needle to pierce the host larva’s cuticle, and paralysing venom is rapidly injected under high pressure. As the ventral vulvulae moves forwards, the egg in the spindle shape cavity also moves forward under an increasingly flat roof. Thus, we can see that the egg is not squashed because the ventral valvulae forms 80% of the egg cavity. The egg is also protected from friction exerted by the dorsal roof, by the enclosing fold of the valvilli. Once the area with the valvilli pass the end of the rhachis on the dorsal valvulae, the 1st and 2nd valvulae open up and separate from each other. The egg is now exposed and is dropped into the space between the 2nd dorsal valvulae and the 1st ventral valvulae, beside the paralysed host larva. Inherent curvature is noted in each of the two ventral valves (mirror image of each other). These curvatures are kept in a straightened position by the olistheter mechanism. So, when they are no longer tethered together, they bend in opposite direction, thus helping to open up the egg cavity and aiding in the release of the egg. It is like the egg is being expelled like seeds from an exploding fruit pod.

Egg production

The eggs are produced in the tubular ovaries known as ovarioles in the abdomen. As they mature, they move down to the oviduct, where it joins the venom duct that drains the venom reservoir. This oviduct/venom duct then penetrates the bulbous end of the 2^nd valvulae between the right lateral process and the median process. It continues into the egg canal, ending as the internal guiding structure. The egg moves forward by the alternating movement of the valves. When two valves move forward and one moves backward, the egg moves forward. The egg is prevented from moving backward by distally pointing micro-structures (ctenidium) on the surface of the egg canal. Egg movement is also aided by lubricating secretion from the accessory glands in the abdomen.

The recovery process, extubation (removal of the ovipositor tube) then follows. See last part of video 1. (from 11 min onwards)

The ovipositor tube is pulled up and coiled inside the re-deployed disc bag. However, for the last bit, the tired wasp simply walks forward, dragging the flexible ovipositor tube out of the drill hole, at an acute angle.

The ovipositor proper (terebra) is now folded back into its resting position, pointing posteriorly. Now the wasp uses both its hind legs to put the two halves of the long sheath back onto the terebra. The big spikes on the inside of the hind legs are now put to good use, completing the job quickly. The wasp can now go to its next drill job.

Lastly, there is one interesting point to note in the video, (7^th to 8^th minute). There is a curious orange-coloured circle on the ventral abdominal (7th) cuticle of the wasp. This is the area into which the ovipositor is bent and pushed into the abdomen. The cuticle within this circle appears rugose (crinkled) in appearance, packing a large surface area in a small space, to be expanded into a 2 cm disc on the next ovipositing event. The darker color of the edge of the circle is due to sclerotization, which strengthens the margin of the circle, preventing the huge stretching force within the circle from tearing the membrane outside it. Another video of a black wasp with a more obvious black circle is available here, (beginning at 30 secs).

Article by Michael Wong

Photo credits:

Fig. 1 and 2 from article in reference 1.

References:

1) Ovipositor of the braconid wasp Habrobracon hebetor: structural and functional aspects. June 2021,