The contrast between the larval stage of insects and their mature adult form is something that we seem to take for granted. Without knowing it, who in their right mind would pick that a butterfly starts its life as a crawling, vegetation chomping caterpillar or that a fly hatches from an egg as a legless maggot before its seemingly magical transformation into one of the masters of the sky?

This is also the case with larval forms of the order Neuroptera. Their fearsome larva bares zero resemblance in lifestyle (apart from being carnivorous – the majority of adult Neuoptera are carnivores) or looks to the delicate, weak flying insect that they metamorphose into.

An adult antlion laying eggs into sandy soil. Photo by Deborah Metters..

Adults in the order Neuroptera superficially look similar to dragonflies and damselflies. For this reason, these two insect orders were originally placed together by the father of modern classification Carl von Linnaeus into the single order Neuroptera. As well as having large, paired, veined wings, Neuroptera (translates from Greek neuron – originally sinew, then later nerve and pteron – wing) all have long slender bodies and prominent eyes, hence von Linnaeus’s ‘mistake’. Neuroptera have longer antennae and different wing venation than dragonflies and damselflies which have since been placed in their own separate order, Odonata.

Within the vast order of Neuroptera is to be found the family Myrmeleontidae, the antlions. The family name neatly translates from its Greek origin myrmex – ant – ants form a large part of the larvae’s diet and leon – lion. The ‘problem’ with the family ‘antlion’ is that the family’s common name is based on the larva, whereas all other families in the order Neuroptera are colloquially named after the adult form – Green Lacewings, Owlflies, Mantidflies to name a few. To follow this same naming convention, antlions should be called Antlion Lacewings.

Antlion larva. Photo by Reiner Richter.

The larva are a beautiful blend of form and function, although appearance wise it is hard to describe them as beautiful. Larval antlions have a fat, bulbous abdomen that is shaped to push backwards through sand and friable dirt, a thorax with 3 pairs of legs and a largish flat head equipped with a fearsome set of large sickle-shaped jaws which are armed with numerous sharp protrusions. The head is attached to the thorax by a mobile ‘neck’ which allows the antlion to rapidly flick its head, which is essential for pit building. They are also covered in forward facing stiff hairs. These allow the antlion to lock its body in place so it can safely subdue struggling prey larger than itself without being pulled out of the bottom of its pit.

The Myrmeleontidae are almost unique in the animal kingdom due to the ingenious pit traps that the larval antlions build. The only other analogous pit-forming animals are found in the obscure, small, family Vermileonidae – wormlions in the order Diptera (flies).

Antlions begin their lifecycle with the female searching for an appropriate area for laying her eggs. Once she finds a spot that looks suitable she lands and tests her choice by tapping her ovipositor on the ground. If she deems the spot appropriate, she inserts her ovipositor and deposits an egg, then continues to repeat this process. After hatching, larval antlions start the search for a suitable place to build their pit trap – their mother has already aided in this process. This is usually in sandy or highly friable soil that is also often sheltered from rain and wind.

Antlion pit traps are commonly seen in sandy soils. Photo by Tony Mlynarik.

As in all things involving locomotion by antlion larva, this search is conducted by travelling backwards due to their forward-facing hairs. Once a suitable location is found the antlion marks out the circumference by excavating a shallow circular groove. It then proceeds to crawl backwards using its body as a plough. Loosened substrate is placed on the animals’ flat head using one of its front legs. This is then tossed out of the way with a quick flick of its head. The antlion continues to move round and round, spiralling its way deeper as it moves towards the centre of its pit. In this way it creates a circular inverted cone with steep sides that are on the verge of collapse. Once the pit is complete the antlion buries itself at the bottom of the cone with just its wide-open jaws projecting out. Now it’s just a case of waiting for some hapless victim, frequently an ant, but also other insects and even spiders, to stumble into the pit.

Due to the pit’s steep sides the prey slides down to the waiting jaws of the antlion which snap shut. If the prey isn’t immediately captured by the antlion, it will find it extremely difficult to crawl out of the pit due to the crumbly texture and steepness of the walls. If this isn’t enough the antlion will also flick material at its victim and through a combination of direct hits or undermining of the wall this invariably results in the animal ending up in the bottom of the pit and in the jaws of the antlion. A combination of venom and digestive enzymes are then injected and the antlion proceeds to suck out the liquefied innards. Once it’s finished feeding the antlion unceremoniously flicks out the dried-out husk and rebuilds its pit.

Adult lacewings, like this Blue Eyes Lacewing (Nymphes myrmeleonoides) lay their eggs above ground, such as on sticks and bark. So, when the eggs hatch, the larvae (which look very similar to antlions) are in clusters, like in this photo below. Lacewing larvae do not make pit traps. In contrast, antlion adults lay their eggs in the ground – the perfect spot for their larvae to make pit traps.
Photo by Reiner Richter.

Over the course of its larval life the antlion will continue to grow and will usually go through three moults. In this process it outgrows its original pit and so needs to move and excavate a new one. This process of growth, moult and new pit production occurs over the course of its larval life. Although their pits are extremely effective, antlions are totally dependent on food coming to them. They also need to maintain their pit, which also requires considerable energy expenditure. They therefore have a low metabolic rate and are able to survive a significant amount of time between meals. The time spent in the larval stage can vary considerably and is largely determined by how often it feeds and grows to be of a sufficient size to pupate.

When a larva is ready to pupate, it digs a small cavity in the soil and then spins a cocoon around itself. Interestingly this silk is produced by the Malpighian tubules which, in the vast majority of insect orders, is a waste excretory organ. So instead of producing silk from modified salivary glands the Neuropterans produce silk essentially from their anus. Also, in the final stage of pupation the antlion is finally able to expel all the built-up waste it has accumulated as, up until this point in time, the gut has been a dead end with no way of voiding waste.

Photo by Donna Tomkinson.

The mature antlion that emerges about a month later from the cocoon is a fragile, feeble flier. The adult form is much larger than the final larval instar. This fragility and size disparity is due to the exoskeleton being extremely thin. They are most active towards dusk, so are rarely sighted, but often are attracted to lights at night. Adult antlions live for around 25 days during which time they feed, mate and in the case of females produce and lay eggs in suitable soil to produce the next generation of antlion.

Article by Tony Mlynarik
Land for Wildlife Officer
Brisbane City Council

Top photo by Rick Franks

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