Mites and Their Hosts

Mites are small arthropods (the order of invertebrates that includes insects, spiders, and crustaceans) related to ticks and spiders. Because of their small size-many are nearly microscopic-mites are particularly well-suited to parasitic lifestyles, and many of them live on other animals. There are many varieties of parasitic mites, and some can be highly destructive to their hosts. Varroa destructor, for example, is a mite that lives in honeybee colonies and feeds on the hemolymph (a bodily fluid that plays the role of blood in insects) of the bees. This mite is thought to be partly responsible for colony collapse disorder, a phenomenon in which honeybee colonies around the world are failing.

Other mites, however, live in commensal or symbiotic relationships with their hosts. In a commensal relationship, one organism benefits without harming the other, whereas in a symbiotic relationship, both organisms benefit from each other. One species of mites, for instance, lives on cockroaches and feeds on the saliva they leave behind after feeding and grooming (cleaning the antennae); this is an example of a commensal relationship. Scientists suspect that many species of feather mites, which customarily inhabit the feathers of birds, have symbiotic relationships. with their hosts. Examinations of the contents of feather mite stomachs have revealed mostly fungal spores, rather than bird cells; scientists hypothesize that feather mites protect their hosts by eating fungi that could otherwise infect their feathers.

In some cases, it is difficult to ascertain whether a mite is a parasite or whether it lives commensally or symbiotically with its host. A number of animals, including several species of bees and wasps, have special flaps of skin called "mite pockets," which are normally inhabited by large numbers of mites. Because the mite pockets cost resources to produce, and because they seem to be adapted to the purpose of housing mites, scientists initially assumed that the mites that lived in these pockets must benefit their hosts in some way. One problem with this supposition is that in many cases, mites fail to provide a benefit that can be directly observed. Some carpenter bees, for instance, have mite pockets but do not seem to suffer any ill effects when their mites are removed. Nevertheless, it is possible that some mites confer a benefit that would not be observable in the laboratory. For instance, mites might very slightly increase their hosts' likelihood of survival or reproduction-such an effect would be significant over thousands of years, but might not be discernible by human observers. Alternatively, the mites might provide protection against some harmful stimulus that is only occasionally present-they might prevent their hosts from being infected by a particular fungus, for instance, that does not occur in laboratory tests.

However, scientists studying the wasp Ancistrocerus antilope made a discovery that posed a more serious problem. Antilope adults have mite pockets, and the wasps are accompanied by mites throughout their entire life cycle. Scientists were surprised to discover that the antilope larvae (young insects) attacked and killed the mites living alongside them. In light of this discovery, scientists have developed an alternative hypothesis about mite pockets. According to this hypothesis, the mites that live in these pockets are parasitic rather than symbiotic. The purpose of the mite pockets is to encourage mites to live in particular areas, where they will do less damage than if they roamed freely around the host. Scientists have pointed to similar mite pockets found on geckos and other lizards to support this hypothesis. The skin in these mite pockets is particularly rich in lymph cells, a preferred food source for mites. It is also unusually elastic and resistant to damage, and it heals more quickly than skin on other parts of the gecko.

Some evolutionary biologists have hypothesized that parasitic mites will tend to evolve to become commensal or symbiotic. Parasitic mites run the risk of killing their hosts, which would deprive them of their food source. Additionally, hosts are less likely to employ defensive mechanisms, like scratching or preening (cleaning feathers), against beneficial or harmless mites. In the future, it may be possible to test this hypothesis by determining when particular species of mites evolved; if the hypothesis is correct, then parasitic mite species will tend to be of more recent evolutionary origin.