The Life of Biting Sandflies

Given that sandflies are the curse of the coastline, it is surprising how little accessible information there is about them. Scientists have studied them on the behest of councils to understand where and when they breed, however these scientific studies do not present much of a window into the life of a sand fly. Being of inquiring mind, I would like to find out exactly where they breed and what their larvae look like so that I can study them for myself. In this post, I will attempt to provide a resource for those who want to study the humble saltwater sand fly.

A sand fly drilling into the back of my finger

There are lots of different types of sand fly and the species are identified by the vein patterns and colour patches in their wings. They belong to the genus Culicoides and C. molestus famously inhabits the canals and sandy shores of South-east Queensland. In the tropical north, C. subimmaculatus is known to breed in muddy mangrove environments. I am not sure which species I have investigated.

As sandflies are so small, a microscope is needed for identification of species. For this blog post, I have only used affordable equipment including a magnifying glass and a set of $20 close-up lenses that screw onto the front of my compact camera.

Sandflies were emerging along the receding water line where beach yields to estuary

Finding where sandflies might be breeding is as simple as squatting down and looking for sandflies walking across the muddy surface. I have observed them mainly at midday to early afternoon, usually walking across the surface toward the wind. In the late afternoon, strong trade winds blow across the ground and both insects and other creatures seem to seek shelter. Usually, I wear long clothes for sun protection and the sandflies do not seem to be biting at any rate in the middle of the day so I can study them unmolested. Most sand fly larvae inhabit a tidal band just below the high tide level of ordinary neap tides and above the level dominated by water breathing crabs and worms.  

Diagram of sandfly life cycle - WikiMedia

Studies have discovered that sand fly emergence occurs mainly during the spring tide part of the cycle. However nothing seems to have been written about the actual process of emergence, let alone any photos or videos taken.  Below is my best attempt so far to video emerging sandflies.  The cover image is a dead pupa that floated to top of one of my collected samples. It appears to be in a clear bag, which may be how the wings are protected.

Observing sandflies in the process of emerging is challenging, even when reviewing video footage things happen so quickly that it is difficult to work out what is happening. I believe a lab study will be needed to sort out what is really happening but this is what I think. As the girl sandflies emerge, they get jumped on by the boy sandflies and there is lots of violence. Alternatively the boys are jumping on the girls when they come in to lay eggs, but the first pattern is common other insect groups. I think that sandflies just pop up out of the mud and can fly a few seconds later. However males sandflies fly can in and land within the space of a single video frame at 30 frames per second. Sandflies suddenly appear and disappear in the footage and much higher frame rate is needed to track their movements. All this action occurs within a few minutes of the ground surface being exposed by the retreating tide. Small trumpeter fish (Therapon jabua) enthusiastically patrol the shallowest of waters and probably eat any sandflies that emerge before the tide has fully retreated.

To speculate about what I think that I have seen, sandflies may be cryptic about how they emerge, mainly to avoid the attention of their conspecifics. Even before they can fly, they appear to jump like fleas when spooked. The big question is where do they expand and harden their wings. A children’s program on sandflies claims that they do this in a bubble in the ground but I can’t find any published info that supports this. What I take to be freshly emerged sandflies seem to have smaller wings and the wings appear to expand over a short period. As the sandflies turn around frequently, they can be clearly visible when facing one direction and nearly invisible when they turn around. I am guessing that they emerge almost ready but need a few seconds to grow and harden their wings. They either jump around to avoid their mates or possibly hide their bodies in muddy pools in micro-depressions. Hiding their bodies in water hides their shadow, which is more visible that that the actual sand fly. In most of the cases when I see a sand fly emerge from the muddy water, running the video backward shows that it was already emerged and moved into the water from somewhere else. Most of these behaviours can be observed in the video, if you are prepared to go forwards and backwards enough times.  Furthermore, sandflies may emerge synchronously. There seem to be small patches of intense activity rather than dispersed trickle of newly emerged insects.

A dead sand fly larva beside the tip a dressmaking pin

Finding sand fly larvae and pupa is even more challenging. The one success I had was to put a few spoonfuls of mud into a small clear drinking water bottle and shake it so hard that the larvae would not have known which way was up or down. The larvae were probably killed by abrasion and when the muddy mixture settled, were in the fluffy sediments between the clear water and the heavier muds and sands. Pouring off the water and some of the fluffy sediment produced a dozen dead larvae.  Sand fly larvae have a head capsule, three small thoracic segments and nine larger abdominal segments.

Scientists have added sugar and golden syrup to sandy samples to make the seawater so heavy that the sand fly larvae could no longer swim down to the bottom of the container and floated to the top. I would prefer to find another method. I have poured the fluffy sediment from another sample of mud into a shallow tray to look for movement but none was seen save for springtails and tiny worms. However later, strange tracks covered the bottom of the tray suggesting that sand fly larva were present but cannot be seen.

Tracks that might be from sand fly larvae

My next attempt was to carefully excise a chunk of mud from prime habitat and use a dressmakers pin to flick crumbs of mud from the edges of my muddy sample until I had worked my way through. Small tunnels can be seen and even followed. I found that the mud was bound together by threads that were strong enough that I could feel their strength. The threads appear to be the rhizoids of a minute but advanced green algae and can be present even when there is no noticeable algal growth on the surface.  Threads within the mud are much finer than any green filaments that can be seen on the surface and are present and apparently alive even tens of millimetres below the surface. It is like discovering a miniature sea grass with tiny leaves and roots has been present in an area I have walked over hundreds of times but never noticed. Despite finding tiny worms below in miniature green meadows, I did not find any sand fly larvae. As a check, I added some water and shook up mud that had been processed and on the following day found a single sand fly pupa.

Sand fly pupa and dressmakers pin

Mysterious threads holding the mud together (click to enlarge)

The threads are from a minute and unidentified green algae

Even in close-up (see baby mudskipper), the algal turf can go unnoticed.

My preliminary research is incomplete. I need to find a simple way of obtaining numbers of larvae. The frustrating search continues, however I did discover an unexpected ecosystem of algal turf and its associated fauna including springtails.

Additional sandfly information is as follows. Sand fly larvae are reported to be predators of small creatures, which I suppose means mainly springtails. The larvae breath via small hook gills at the end of their abdomen. Only female sandflies feed on blood. Mudskippers are their main victims although wading birds cop it too. Adult sandflies live only 2-3 weeks. Swarms are often visible in patches of sunlight in the lee of trees or other wind blocking objects. The swarms are reported to be where the boy and girls get together.

Control of sandflies is difficult as they live in estuaries which contain other biodiversity which we do not want to kill with chemicals. Fogging may be used when sandflies are severe but fogging has no persistent effect. Within their muddy habitat, sandflies may be predated by tiny robber flies and hover flies. Rove beetles also swarm over sand and mud surfaces and may be predators of sand fly sized prey.