One of the first things I did right after launching my “Meet Your Neighbours” gallery was to ask people which photo draws their attention the most while scrolling down the page. Responses varied, but one in particular was quite dominant: “violin larva”.
The name “violin larva” is not official, but it does great service to describe the insect in question – larva of the threadwing antlion (Dielocroce hebraea). Ever since I read Piotr Naskrecki’s post about these insects I knew I was going to write a post about their larvae. I feel like this is a classic insect one get to see when taking an Entomology course in Israel, but outside the Middle East and Africa not too many people are familiar with them.
Threadwing antlions are interesting animals. They belong to family Nemopteridae, and they are unmistakable in appearance – adults have extremely long hindwings, like thin threads or ribbons, giving the insect its name and the appearance of a delicate fairy when it is in mid-flight. Their larvae are also quite unique for having an extended prothorax (long neck), unlike the bulky larva characterizing the other neuropteran families. Nemopteridae contains two subfamilies: Nemopterinae (spoonwings), in which the day-active adults have wide, ribbon-like hindwings, and the strictly-nocturnal Crocinae (threadwings), containing adults with narrow, thread-like hindwings.
Members of subfamily Crocinae have a narrow habitat preference. They are found in arid desert zones and prefer caves or rock shelves sheltering thick dust patches of fine clay. The larvae are ground dwellers, taking advantage of the fine clay for camouflage. They are voracious predators, using their sickle-shape mouthparts to inject venom into their prey in order to paralyze it. They are characterized by a relatively long neck, almost a half of their total body length. The neck may look like a strange adaptation for life on the cave’s floor, but has several functions.
These giraffe-like larvae are sit and wait predators that hide under the fine sand in ambush for passing insects. Here is a video showing the predation of a silverfish by the threadwing larva. See if you can spot where the larva is located before it pounces on its prey:
After watching the video it becomes clear that the long neck may assist in protection from a struggling prey that can damage the larva’s soft abdomen. The variety of arthropods that these larvae feed on include venomous assassin bugs and spiders. But how do the larvae hide themselves so well in the sand? Watch a short timelapse of the process:
The neck also comes in handy when the larva buries itself in the soil. Notice how the larva first checks the area with its head to make sure it is not taken by another already-hidden larva. If the larva’s abdomen is bitten during burrowing by another larva, this can be fatal. The sight of larvae fighting over a spot for burrowing is common, suggesting that they are somewhat territorial, therefore they try to avoid conflict by examining the area before burrowing.
Development of the threadwing larvae is slow, and larvae can spend up to two years living in the fine sand of the cave. Once they complete their development they construct a tiny spherical cocoon from sand grains glued with silk, in which they pupate.
The curled hindwings of the pupa somehow remind me of a butterfly’s proboscis (only there are two, and they are curled backwards). The head bares two “horns” that assist in puncturing a hole in the cocoon, allowing the adult to emerge to the outside world.
Adult threadwings have elongated heads, with the ventral part extended backwards giving it the appearance of a duck’s bill (some people refer to them as duck-faced antlions, I suggest the name duck-faced fairies though). They rarely feed as adults, although they are sometimes seen drinking dew. Threadwing antlions often rest on spider web found on the cave’s ceiling. It is unclear how they avoid getting tangled by the web. They are so lightweight that they do not apply too much pressure on the silk and thus can rest even on loose strands.
Finding threadwings in the wild is not an easy task. You must first find the right type of habitat, and even then nothing guarantees their presence. While I cannot say these insects are common, it is always a treat to stumble upon them. To me they are the closest thing to legendary fairies, hiding a dark secret of their early life as little giraffe monsters.