by Matthew Cobb
Darwin once wrote to a friend “I cannot persuade myself that a beneficent and omnipotent God would have designedly created theIchneumonidae with the express intention of their feeding within the living bodies of caterpillars.” These creepily amazing solitary wasps lay eggs in or on other insects; the hatched larvae then proceed to devour their prey from within, generally keeping all the vital functions going while they are about it.
Some of these amazing wasps have recently been captured on video (300 fps!), attacking various Spanish ants that were going about their business. The films form part of a paper by Cees van Achterberg and José Maria Duran published in the open access journal Zookeys.
With incredible dexterity, the tiny wasps (less than 2 mm long) managed to lay eggs in their hymenopteran cousins, just slipping their ovipositor between the plates on the ant’s gaster. At the end of the first film, one of the ants manages to grab her tormentor in mid-air and nom her, as we scientists say. So yes, the ants most definitely know the wasps are there and do their best to avoid been attacked.
The first film shows the wasp Elasmosoma luxemburgense ovipositing in Formica rufibarbis ants. There’s no soundtrack. I happened to be listening to Pergolesi’s Stabat Mater ‘O quam tristis’ when I watched them, which is recommended for a suitably grim accompaniment.
The second film shows Kollasmomsoma sentum attacking Cataglyphis ibericus:
The authors clinically describe the behaviour of four different species of wasp, and their precise adaptations for attacking their prey. Here’s the section on Elasmosoma luxemburgense:
The wasp attacks always come from behind, paralleling their longitudinal axis to those of the ants. When they are less than 1 cm from an ant they dart forward and the fore legs contact the dorsal surface of the metasoma first. Meanwhile the hind legs, arranged in curved shape, are situated to brace the apex of the metasoma (Fig. 2).
Contact with the fore legs is usually followed by hitting of the parasitoid’s head on the host’s metasoma. At this moment the middle and hind legs grasp the metasoma and the wasp folds its wings. The site chosen by the wasp for the initial hit of the fore legs, or the head, is usually the posterior margin of the first gastral segment (T1; Fig. 3), i.e., of a total of 48 hits observed, 44 were on the posterior margin of the first gastral segment (91.7%), three on the posterior margin of the second (6.3%), and one on the posterior margin of the third (2%).
When the hit occurs at the posterior margin of the second or third gastral segments, the wasp climbs onto the metasoma, changing its position to reach the posterior margin of the first gastral segment (Fig. 4).
This locational preference for alighting may be visually stimulated by the differentiated border of the posterior margin of the first gastral segment, enhanced by the characteristic dark stripe behind it. The frame analysis in the film clip suggests that the wasp’s head hits the posterior margin of T1 with the mandibles opened, and that a slight deformation of the suture between T1 and T2 is produced. Presumably, the modified structure of the T1-T2 suture is used by the wasp to secure its grasp. The tarsal modifications of Elasmosoma (vestigial tarsal claws and enlarged pulvillus; Shaw 1985, 2007) may be adaptations to effect this grasping behaviour. In the final arrangement, prior to oviposition, the fore tarsi usually grasp the posterior margin of the first gastral segment, and the hind tibiae and tarsi brace the apex of the metasoma on the fourth gastral segment, with the middle legs positioned near or somewhat posterior to the hind margin of the second gastral segment (Fig. 5).
This arrangement of the legs facilitates the appropriate position of the wasp’s metasoma in order to insert the ovipositor into the posterior area of the last metasomal segment, between the pygidium and the hypopygium, probably through the anus. Poinar (2004) dissected the metasoma of the ant Formica obscuriventris clivia Creighton, 1940, a host of Elasmosomamichaeli Shaw, 2007, and found for the first time the wasp egg “just under the body wall of the ant’s metasoma.”
The existence of parasitoid wasps posed a real problem for people trying to work out what the relationship was between caterpillars, pupae and butterflies/moths. I wrote about this in my book The Egg & Sperm Race (aka Generation in the US):
Johannes Goedaert (1617-1668) was an artist from Middleburg in the Dutch Republic, who often included butterflies and moths on his flower paintings. But as well as representing the striking form and colour of these insects, Goedaert tried to understand their natural history. Throughout his life, Goedaert was obsessed with tracking the transformation from egg to caterpillar to pupa to adult. He carefully collected larvae and caterpillars from the Zeeland fields, brought them home, did his best to find appropriate food to keep them alive, drew the caterpillar, noted when the insect began to pupate, drew the pupa, then waited to see what would emerge. This sometimes took several months, after which he drew the adult that he found fluttering, buzzing or scrabbling around in the container.
In 1662, after 45 years of observations, but with no scientific or academic training, Goedaert published Metamorphosis et Historia Naturalis Insectorum (“The change and natural history of insects”). (…) In 236 pages and 80 hand-coloured plates, Goedaert showed larvae, pupae and adults. In 1667 and 1668, two further volumes followed, containing a further 126 coloured plates. The stunning illustrations in Goedaert’s book, backed up by a series of observations (…) showed there was a link between the two stages of the life-cycle of many insects – the larva (or maggot or caterpillar) and the adult. However, although Goedaert realised there was a consistent relationship between the two stages – for example, each kind of caterpillar produced a specifically shaped pupa which went on to release a specific kind of butterfly – he did not think that they were the same organism with different shapes. Instead, he accepted the generally-held view that the adult was generated from the decay of the caterpillar.
To his surprise, however, Goedaert frequently found that a given kind of caterpillar did not always produce the expected butterfly – sometimes a caterpillar would generate dozens of flies or wasps (“out of one and the same Species of Catterpillars, a Butterfly is produced, and 82 Flyes”). Goedaert was nonplussed by this apparent violation of the systematic transformation of each kind of caterpillar into a specific type of butterfly; he felt that it was “against the usuall course of Nature, that from one and the same Species of Animals, an Offspring of different Species shou’d be gendred”.
We got a similar surprise at the Manchester University Zoology Xmas party in 2007. As usual, the students had to dress up as their favourite animal. Some of the girls came as a caterpillar, which was reasonably impressive: