letting nature back in

Many species of fly are pollinators, a role for which they receive little common recognition. Perhaps also not so well-known is that some species of flies indulge in bubble-blowing.

A few months ago I was looking at the tiny flowers on a Tassel-berry tree (Antidesma venosum) in our garden. As the common name implies, the most noticeably characteristic of the tree are the berries, which hang down in long tassels. These berries are preceded by less conspicuous pale flowers. I was surprised when I noticed that the flowers were being visited by several different kinds of flies, apparently seeking nectar and pollen.

Showing off its colours, this male blow fly sucks up nectar from the flowers of the Tassel-berry tree in our garden

Blow flies are part of the Calliphoridae family of flies (Diptera). Typically, most species of blow fly lay their eggs on fresh carcasses. The eggs hatch into larvae (maggots) that eat the flesh as it starts to rot. The maggots of some species of blow fly have been used in human medicine, to help clean wounds by eating the surrounding decayed flesh.

Also beneficial to us humans, blow fly maggots are useful forensic indicators and they assist in speeding up the necessary process of decay. A recent research paper reports that blow flies play a major role in the pollination of fruit orchards, including mangoes. Blow flies have the advantage of being cheaper and easier to raise and maintain as orchard pollinators than, for instance, honeybees (Saeed et al. 2016).

Once larval growth is complete, blow fly maggots burrow into the soil to pupate, and in the next part of the cycle emerge as adult flies. In their adult form, blow flies eat flower nectar, plant sap and other sugary substances, and in the process of visiting flowers in search of food they act as pollinators.

Pollinating flies busily foraging in the tiny flowers

Also feeding from the flowers on the Tassel-berry tree were flies from the large and diverse Tachinidae family. All Tachinid flies are endoparasites, that is in larval form they live inside the bodies of their hosts, usually the caterpillars (larvae) of butterflies and moths. Female Tachinid flies lay their eggs directly into or onto the bodies of the larvae of the host species, or they lay their eggs on the leaves that the host caterpillars eat. The fly eggs hatch inside the host larvae. Most hosts die as a result of this, and once fully developed the Tachinid fly larvae go on to pupate in the soil.

Like the blow flies, adult Tachinid flies are often found visiting flowers, eating nectar, pollen or honeydew. The characteristic bristles at the end of the abdomen can be seen in this photo

Tachinid flies significantly control the numbers of insects that they parasitise, including insects that feed on agricultural crops and on garden plants. Some gardeners actively encourage Tachinid flies, regarding them as “natural pest controllers”. By controlling species that can cause significant agricultural crop losses, Tachinid flies may play an important role in the economy as biological control agents.

Many insects, including many species of fly, indulge in something called bubble blowing. These bubbles are not airy bubbles; they are round droplets of previously ingested fluids like nectar. There are various theories as to why some insects do this, with the most prevalent theory being that the process aids digestion.

A Tachinid fly sitting motionless on a leaf of the Tassel-berry displaying a bubble-like fluid droplet

Flies usually get a negative press, so reiterating their detrimental aspects here seems redundant. Instead, I am interested to share something about the incredible diversity of flies, and that the natural functions they perform are surprisingly complex, and, from a human point of view, often beneficial too.

Sources: Blow Fly. Insects. http://www.everythingabout.net/articles/biology/animals/arthropods/insects/flies/blow_fly/;  Parasitoids: Tachinid Fly (Diptera). [n.d.] Home and Garden Information Center, College of Agriculture and Natural Resources, University of Maryland Extension.  https://extension.umd.edu/hgic/insects/parasitoids-tachinid-fly-diptera;  Saeed, S., Naqqash, M. N., Jaleel, W., Saeed, Q., & Ghouri, F. 2016. The effect of blow flies (Diptera: Calliphoridae) on the size and weight of mangos (Mangifera indica L.). PeerJ, 4, e2076. http://doi.org/10.7717/peerj.2076;  Scholtz, C.H. and Holm, E. 1985. Insects of Southern Africa. Durban: Butterworths.

Posted by Carol