Collections technician and PhD student Shaun Thompson has spent several years working with curators Julia Kasper and Phil Sirvid to study one of the most unusual and elusive groups of flies in Aotearoa New Zealand. He aims to create comprehensive species descriptions and distribution records of the Acroceridae, or “small-headed flies”, of New Zealand. Here he shares what he has learned about these strange flies.
The Acroceridae, commonly known as “small-headed flies”, are a group of flies widespread throughout the world. Despite being widespread, they are rarely encountered, and little is known about them. Many members of the group have unusually small heads, which gives them their common name. As adults, many species of Acroceridae feed on nectar and have important roles as pollinators.
What makes the Acroceridae remarkable is that their larvae are endoparasitoids of spiders, meaning they develop inside spiders. After the eggs are laid, the larvae hatch and begin to seek out spiders. Although they are blind, they are very sensitive to movement and will inch towards spiders. Once they reach a spider, they’ll climb up it and insert themselves inside, often through the abdomen, and eventually settle in the spider’s book lungs. The larvae will then spend many months developing inside the spider before exiting to pupate into an adult, which kills the spider.
Who’s here? The Acroceridae of New Zealand
In New Zealand, there are three groups of native Acroceridae: Apsona, Helle and Ogcodes. The first group, Apsona, has only a single species, Apsona muscaria. This species is coloured a spectacular metallic blue and is covered in long hairs, giving it a fluffy appearance. They also have a long proboscis (straw-like mouthpart) that is used for feeding on nectar. Although they are very rare throughout most of New Zealand, they can be remarkably abundant in subalpine habitats such as Arthur’s Pass, a biodiversity hotspot. Their larval host is unknown, but it is probably a primitive group of spiders, such as the tunnelweb spiders.
Helle includes two species: Helle longirostris, which is widespread throughout the country, and Helle rufescens, which is only known from the north-west of the South Island. Both species have a large thorax, which gives them an odd-looking “hunchback” appearance. Like Apsona, they also have a proboscis, which they use to drink nectar.
The last group, Ogcodes, has seven species native to New Zealand (and numerous species in other countries). Unlike Apsona and Helle, they don’t have a proboscis and are unable to feed as adults. Females in this group have unusually small heads, whereas the males have regular-sized ones. Species in this group parasitise flower spiders (Diaea) and desid spiders (Matachia), but it is likely that they have many more hosts waiting to be discovered. Of these species, only Ogcodes brunneus is common. It is widespread throughout New Zealand and can be unusually abundant in grassy wetlands.
Very little information was available about those three groups of New Zealand’s Acroceridae. Most species were only known from a handful of specimens, and even professional entomologists struggle to reliably identify them. It has also been speculated that there are numerous undescribed species present. Other aspects of their biology, such as distribution, habitat preferences, and floral associations, were poorly understood.
The search for small-headed flies
My PhD focused on revising the taxonomy and documenting the diversity of New Zealand’s spider flies to make identification and biological information more accessible. To conduct my research, I first needed to find the flies. This involved travelling to many places throughout New Zealand, where the flies haven’t been recorded. Some of the places I visited included Arthur’s Pass National Park, Mount Taranaki, Stewart Island, and the Coromandel Peninsula, all of which yielded interesting discoveries.
Finding the flies in the field was a tricky process. There was limited time, as Acroceridae are only present as adults during summer months. They are also difficult to find in the field, due to their small size and often patchy distribution. However, searching for flies in the field on its own would not be able to provide enough specimens for this study. Fortunately, Te Papa and other museums in New Zealand hold large collections of Acroceridae that they kindly allowed me to loan to study in detail.
How are they analysed?
With the extensive dataset of Acroceridae specimens I put together, I am now creating a taxonomic revision, a series of updated species descriptions for the whole group. In this revision, every known specimen of each species has been used to uncover their distributions and to create detailed descriptions and step-by-step instructions for identifying them. These species are supported by a combination of morphological and genetic data, which is the first time it has been used on these species in such a manner. Genetic sequencing has been used alongside morphological comparisons to clarify species designations and reveal patterns of divergence within this group.
What was discovered?
Despite extensive travels throughout New Zealand, I ended up finding an undescribed species in Wellington’s backyard. While searching in the hills of Eastbourne, I encountered males of Helle tigris, a new species which I am naming for its tiger-like stripes. I had seen female specimens of this undescribed species in museum collections but had never seen one in the field or alive before. From further study, it appears that this species occurs sparsely in a wide range across but the North Island.
The new species Helle tigris can be distinguished from other Helle by its orange stripes and shinier thorax. Only a few specimens have been found, but they seem to favour healthy native forests, such as the one in Eastbourne. They’ve been found visiting flowers alongside Helle longirostris, which are more abundant and far less picky about their habitats.
The taxonomic work has also been supplemented by biological data about all the nectar-feeding habits of Apsona and Helle. Until now, the flower preferences of these two groups were largely unknown. Because pollen sticks to them when feeding, pollen grains could be extracted and identified by a palynologist (person who studies pollen) at Massey University. This was even possible for older museum specimens. The results unveiled which flowering plants these groups visited. It was found that they both visit a wide range of species, but they appeared to prefer Asteraceae, a group that contains daisies. These records, combined with direct observations, highlight the importance of the Acroceridae as flower visitors in their ecosystems.
Protecting our small-headed flies
Lastly, this revision is being used to inform the threat classifications of these species. In New Zealand, threat classifications for species are assigned by a panel appointed by the Department of Conservation. For flies, there has only ever been one such panel, which was convened in 2012. A second panel, in which Te Papa staff are involved, is currently being organised. This panel aims to update the threat classifications of many of New Zealand’s fly species, including Acroceridae. Once complete, these threat classifications will inform conservation decisions made by DoC, which will help to protect New Zealand’s taonga species.
The small-headed flies uncovered
By combining fieldwork, museum research and modern analytical techniques, this work has discovered much about our small-headed flies. From the New Zealand wilds to Te Papa’s specimen drawers, uncovering the lives of our small-headed flies demonstrates how much there is left to learn – even in our own backyards.
Interesting blog Shaun. Could you tell if the plants visited were native or exotic (or both), or was the resolution not good enough with the pollen?
Hi Lara, the resolution was quite mixed, but those with good resolution were mostly native plants.
Thank you for these informative emails. I live in Rotorua and it is more than 50 years since I visited the National Museum in Wellington. It is really good to keep up to date with your work