Parasitism among arthropods

Insects, like all of us, live in a world of limited resources and try to claim their share. If another organism tries to also use the same, already claimed and often processed resource, we call it parasitism. Parasites can be thieves of provisions (klepto-parasites), invade a host's body (endo-parasites) or just suck its heamolymph (ectoparasites) share the meal of a predator (commensals), smuggle their offspring into a nest (brood parasite), hitch a ride to food .... Parasite host interactions are always interesting and full of complex strategies and behaviors because of course the host tries to prevent the parasite from reaching its goal. But in several cases, surprising symbiotic relationships have evolved over time. As usual, I tackle the topic in its broadest possible form. 

It seems that wherever there are insect eggs or larvae, there are also parasitic wasps around. Here a Platygastrid inspects the eggs of Chelinidea, a cactus bug.

The ichneumon Rhyssa persuasoria is able to find wood boring larvae under inches of wood and determine if they are suited as hosts. She then pushes her incredibly long, thin ovipositor through the wood to place her egg in the wood borer. Some ichneumonids pupate within the host, so the pupa appears draped in the skin of the larva that the wasp larva parasitized.

When I was trying to raise a big caterpillars in Germany as a kid, hundreds of parasitic wasp larvae emerged from it and pupated on the outside of the dying caterpillar. I was so shocked that I did not try raising any butterflies from caterpillars for a very long time. Nowadays, having worked with Dave Wagner, I know a save way to get healthy caterpillars: catch a pregnant female, wait for her to oviposit and raise clean caterpillars from those eggs. Of course, that does not help if you need to solve the mystery of an unidentified caterpillar found in the wild.

 I collected fat green caterpillars from my winter lettuce here in Arizona, and trying to raise them, I invested a lot of extra lettuce leaves, but got only tachinid flies for my efforts. One or rarely two flies hatched per caterpillar and they emerged only after the lepidoptera pupa was formed. The typical barrel shaped fly pupa was found next to the lep pupa which had a big hole at one end.

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Tachinid eggs can often be seen firmly attached to adult beetles. The development of the fly has to be quite fast to be completed within the lifespan of this relatively short-lived host.

The big scoliid Campsomeris ephippium seems to be a recent arrival from Mexico. We have been seeing them only since  about 2012 in SE AZ. Their hosts are the grubs of our big scarab beetles like Dynastes. Likely scenario: wasp locates a grub, digs down to it, stabs it to paralyze it, and lays an egg on it: Idiobiont parasitoids.

Many wasps provide paralyzed hosts for their off-spring, the most well known is probably the Tarantula Hawk. But many species in several different families of wasps follow the same pattern, all very host-specific and often with extreme athleticism and surprising orientation capacities. They dig their nests, then go out to find prey, bring it back, lay their egg and close the nest entrance, patting the sand back into place with a rock held between their mandibles.

But even these parasitic wasps have to worry about super-parasites themselves: 
while Ammophila is stashing her big caterpillar into the prepared hole, a little cuckoo is watching. When Ammophila drops her guard for a second, the little Argochrysis wasp slips in and lays her own egg. The egg of the small cuckoo wasp hatches first and probably eats the ammophila egg as well as the caterpillar.

Brood parasitism is extremely common in hymenoptera that provide for their offspring but do not stay around to guard the eggs and larvae. Solitary bee nests produce any number of Bee Flies, Blister Beetles, Wedge-shaped Beetles and Velvet Ants. But may parasites or cuckoos are actually closely related cousins in the same family of bees, for example Megachilidae.

But even ants, who care for their offspring in the most devoted altruistic way have uninvited guests: Several beetle families like weevils, carabids, Bess Beetles and scarabs have members that are adapted to living with ants as nest parasites. They somehow get around the ants' vigilant guards and then feed on the ants' food stores or even their brood.
This anteater scarab, Cremastocheilus mexicanus was flying around some ant hills on Kitt Peak road. In typical fashion, it crash landed and played dead, with legs sticking up ... Now the ants should have come out, ca...rried her into the colony like prey... but once in there, the beetle somehow avoids getting eaten by the ants. I have noticed that she can fold all extremities and even her head very tightly against her body, with no attack point for the ants sticking out. My theory is that this protects her initially against what the ants usually do to beetles: they tear them apart, pulling from all sides. After some time in the ant nest, the beetle might have absorbed enough of the nest-perfume to blend in and for the ants to ignore her, at which point she lays eggs that produce larvae that feed on the rich food supplies and also on the brood of the ants.

probably Lasius (subgen. Acanthomyops) latipes

 But why just sneak in and feed on food storage and brood if you can capitalize on the entire colony, resources, structures, workers and all.  Queens of one of the parasitic subterranean Lasius species   sneak into nests of other Lasius species like L. alienus or L. neoniger, kill the resident queen, and use the sizable worker force to start raising her own brood. After a couple years, the worker force will have completely turned over to the parasitic species. Colonies are entirely subterranean - you will rarely ever see workers above ground - they tend scale and aphids on plant roots. Acanthomyops ants are not uncommon but they are patchily distributed. They have an alarm pheromone that smells strongly of citronella. Id and info by Alex Wild

There may be more parasitic wasp species than any other group of  parasites, but wasps are also  hosts for a very interesting order of obligatory parasites the Strepsiptera (Twisted-winged Insects):  As I understand it, Taxonomists were not quite sure where the order of Strepsiptera would fit into the phylogenetic tree, but the current understanding is that they are closest to coleoptera (beetles). Their bodies seem highly modified for their lifestyle as endoparasites. At least larvae and females live for the most part of their lives inside the bodies of other arthropods, often in hymenoptera. The short lived winged males fly and find females to mate top left photo). The eggs hatch within the female, who never left  the host, and the young larvae move out from her body cavity to find new hosts.

Many arachnids are exoparasites,  like ticks and many mites. Those often switch from  parasitism as larvae to a predatory life style when mature.  Even those darned chiggers fall into this category. In the picture a  Cobweb spider Euryopis sp. carries a trombidioid larva while a trombidioid adult mite is striking out on its own.

Mites on a carrion beetle are NOT parasitic as in feeding on the beetle or even stealing his food when he carries them to a delectable little corpse. This is an example of phoresy (one animal attached to another exclusively for transport). But the story is interesting, if disgusting to some: The beetle and his mate will bury the dead bird or mouse and masticate it into a ball of food for their larvae. They'll actually stay with their brood and care for them. But before the beetles got a hold of that prize, other insects have already laid eggs on it, for example flies. Supposedly, those phoretic mites,  deutonymphs in the genus Poecilochirus, will destroy that competition, feeding on eggs and small larvae of flies. But being predators, the mites are also a threat to the beetles' own offspring, maybe that's why the beetle parents keep such close watch?
Similar mutualism exists between mites and dung beetles.

Here are more phoretic hitchhikers:  Peudoscorpions, being flightless arachnids and very small, use big strong flighing beetles as public transportation.  Trichocnemis spiculatus neomexicanus is over 2 in long and the pseudoscorpions were hidden under its elytra. But I have also found them clinging to feet and even antennae of beetles, and that probably slows smaller longhorns down considerably.

Avery special case of cohabitation and phoresis was just posted by Alex Wild on Face Book: Attaphila, a tiny cockroach living in Atta (Leafcutter Ant) nests and travelling with young alate queens to their new colonies. See the link here

When predators cannot enjoy their meal in peace, but are joined by flies that are drawn to the smell of the slaughter we call that comensalism - those flies are co-eaters. Here an Assassin Bug (Pselliopus sp.) killed a Mason Wasp (Eumenidae) and a number of  Milichiid flies invited themselves to the feast.

These are only a few examples of the many types of parasitism that can be observed in the insect (or spider world). It is a very complicated and fascinating field.