Antlions

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This Wayne's Word Noteworthy Plant Originally Published In July 1997

Antlions: Denizens Of The Sand

© W.P. Armstrong 2007

The arid southwestern United States is rich in diverse forms of animal life, but there is one little creature that looks like something out of a science fiction horror movie. In fact, a remarkably similar large scale model of it was used in the Star Trek II film, "The Wrath of Khan!" This remarkable creature makes funnel-shaped, crater-like pits in soft sand, and then waits patiently at the bottom to ambush a hapless passer-by that happens to fall in. It is commonly called an antlion, referring to its habit of preying on small crawling insects, such as ants. If antlions were about 100 times larger, they would be a formidable threat to people walking in the desert.

The actual creature waiting at the bottom of its ingenious pitfall trap is the larval stage of an unusual insect. This larval stage is often called a "doodlebug" by certain people. In fact, some of these people claim that repeating the phrase "doodlebug, doodlebug, come out of your hole" with your face close to the ground will cause the antlion larva to crawl out into view. The Wayne's Word staff has tested this hypothesis repeatedly and we must agree with the conclusions of other scientists that antlions do not understand the word "doodlebug." In fact, they can't understand anything you say. When your speak close to their crater, some grains of sand become dislodged and fall into the pit. Falsely believing it has made a capture, the antlion becomes excited and crawls out into view. This same behavior can be observed in various types of spiders by gently touching their web with a small twig or feather.

Crater-like pits made by the antlion larva in soft sand. Each funnel-like crater is approximately 2 centimeters in diameter and almost as deep.

Antlions are members of a large order of unusual insects, the Neuroptera. They are often called the "nerve-winged insects" because of the elaborate pattern of longitudinal and cross-veins (nerves) in the four wings of adults. These insects undergo complete metamorphosis with an egg, larva, pupa and winged adult. The larval stage is typically a grotesque, wingless creature with long, sicklelike jaws. Pupation usually occurs in a silken cocoon; however, the silk is not derived from modified salivary glands as in most insects, but is produced by the Malpighian tubules and is spun from the anus. The order includes many predaceous, night-flying species, including lacewings, alderflies, snakeflies and dobsonflies. The immature or larval stage of dobsonflies, called hellgrammites, are familiar to fishermen and fisherwomen because they are commonly used as bait. One curious member of the order, called a mantispid, shows a striking resemblance to a miniature preying mantis with enlarged, grasping front legs.

Antlions belong to the family Myrmeleontidae and include over 600 described species. Two of the most common genera in the southwestern United States are Myrmeleon and Brachynemurus. Like many other members of the order, adult antlions are commonly seen around lights and campfires, particularly during the late summer and fall. They have two pairs of long, narrow, many-veined wings and a long, slender abdomen. Although they greatly resemble small and unrelated dragonflies, called damselflies, they belong to an entirely different order of insects.

The adult antlion can be readily distinguished from a damselfly by its long, clubbed antennae. It is a feeble flier and flutters about through the night air in search of a mate. The adult does not feed and has a relatively short life span of of 20-25 days or longer (up to 45 days). As in all animals, without the mating imperative, the genes of this remarkable species would be lost forever. The most incredible part of its life cycle begins after the gravid (pregnant) female lays her eggs in the sand, and after the immature larvae hatch from the eggs.

Dorsal view of adult antlion in resting position.

Lateral view of adult antlion in resting position.

The antlion larva is a ferocious-appearing creature with a robust, fusiform body bearing three pairs of walking legs and a slender neck. Its small head bears an enormous pair of sicklelike jaws (mandibles) with several sharp, teethlike projections. Like sharp hypodermic needles, the hollow jaws pierce the victim and suck fluids out of its body. According to R.E. Hutchins (Insects, 1966), the jaws are capable of injecting venom which digests and dissolves the body contents of the prey.

Close-up view of an antlion larva showing its peculiar (grotesque) fusiform body and enormous, toothed jaws (mandibles).

In some species, the larva excavates a conical pit in the sand by crawling backwards in circles, at the same time flipping out sand grains with its long jaws. As it moves round and round, the pit gradually gets deeper and deeper. Eventually the crater reaches 2-4 centimeters across and almost as deep, with very steep walls. The slope of the funnel is adjusted to the critical angle of repose for sand, so that the sides readily give way under the feet of a would-be escapee. The larva waits quietly at the bottom of the pit, with its body off to one side and concealed by the steep wall. Only its sicklelike jaws protrude from the sand and often they are in a wide-opened position.

When crawling insects, such as ants, inadvertently fall into the pit it is virtually impossible for them to climb the loose sand on the steep walls. To make matters worse, the antlion quickly flips out more sand, thus deepening the pit and causing miniature landslides along the walls which knock the struggling ant to the bottom. If the ant or other insect is large enough it may escape, but usually its struggle is hopeless when it is seized by the powerful jaws of the antlion. Antlion larvae are capable of capturing and killing a variety of insects, and can even subdue small spiders. Often the struggling victim is pulled beneath the sand as its body fluids are gradually siphoned out. After consuming all the contents, the lifeless, dry carcass is flicked out of the pit, and the pit is readied for a new victim.

Antlion pits are very common in sandy washes and riverbeds. They are especially abundant in soft sand beneath trees, such cottonwoods and willows, or under overhanging rocks. In some places they are so numerous that they resemble miniature craters on the surface of the moon. Apparently the larvae prefer dry places that are protected from the rain.

The larvae are relatively easy to catch and maintain in captivity. Although they look ferocious, they pose no serious threat to humans. They are much too small to bite your fingers or drag you into their sand pit. When disturbed, the larva usually remains motionless. It is covered with a layer of dust or sand and is easily overlooked. All you need is a container of soft, dry sand and an ample supply of small, crawling insects for food. We have raised antlion larvae in glass bowls at Wayne's Word and have observed all the intimate details of their entire life cycle. We once had an entire antlion "condominium" in a large plastic tray, that is, until a cat decided to use it for a litter box. Antlions will also be at home in a bowl of sugar or salt, and make their usual circular pits. Sometimes they tend to get a little messy as they flick sugar all over the table top. It is very startling to guests when they reach for a sugar bowl and find a grotesque creature at the bottom flipping grains of sugar at them.

Crater-like antlion pits in the sugar bowl at Wayne's Word.

Eventually the larva attains its maximum size and undergoes metamorphosis during which it transforms into a winged adult. The entire length of time from egg to adult may take two or three years. According to Hutchins (Insects, 1966), this unusually long life cycle may be attributed to the uncertainty and irregular nature of its food supply. When it first hatches, the tiny larva specializes in very small insects, but as it grows larger, it constructs larger pits and catches larger prey. When it is full grown, the larva constructs a spherical cocoon of sand grains cemented together with silk. The cocoons of a common species of the southwestern United States are about the same size and shape as large rabbit droppings, and may be buried several centimeters deep in the sand. How the larva accomplishes this under the sand without getting sand grains inside the cocoon is rather remarkable.

Antlion cocoons resemble rabbit droppings covered with sand grains. They are often buried several centimeters deep in soft sand and are difficult to spot when you attempt to excavate them.

Close-up view of an antlion pupa inside its sand cocoon.


Hanging Out At Bars & Hilltopping To Find Mates

When the cocoon is completed, the larva transforms into a pupa where it remains for several weeks (or months). Finally the pupa breaks through the wall of the sand cocoon and pushes itself up to the surface. A beautiful, winged adult antlion crawls out of the pupal case and soon flutters away in search of a mate. Since they are active at night (nocturnal), it is not always that easy to find a mate in the pitch darkness of summer and fall.

Adult antlions are attracted to lights, and their odds of finding a mate may increase on a neon sign or window screen. It is interesting that some insects that are active during the daytime (diurnal), such as butterflies, find their mates by a phenomenon known as "hilltopping." They simply fly upslope to the summit of the nearest hill. In San Diego County, California during the summer months it is common to find dozens of species of butterflies (including beautiful tiger swallowtails) chasing each other in rather erratic flying behavior at the tops of chaparral-covered hills. These remote mountain tops are popular meeting places for sexually active insects looking for mates.


The adult antlion superficially resembles the unrelated damselfly. It can readily be distinguished by its longer, clubbed antennae and nocturnal life style.

Antlion larvae collected in southern California in September formed their distinctive sand cocoons the following June and July, and winged adults began to emerge in August. During this time they were fed literally hundreds of ants, flies, silverfish and spiders, all collected at the headquarters at Wayne's Word. At times the staff wondered if they would ever stop eating and form their cocoons, especially since we were running out of food for them. Although we had a plentiful supply of Argentine ants (see Wayne's Word Volume 4, Fall 1995), we found that this well-adapted little ant could sometimes escape from the sand pits, only to find food caches in the Wayne's Word kitchen cabinets, often returning with thousands of fellow food foragers.

The use of pitfall traps by primitive cultures dates back many thousands of years in Africa, and yet, antlion larvae have been using this method of capturing prey for millions of years with virtually little or no change. Like other fascinating creatures, their instinctive behavior is genetically programmed, each new generation knowing exactly how to perform seemingly impossible tasks with precision and artistic beauty. Antlions are particularly remarkable with ingenious pitfall traps and their clever method of outmaneuvering prey by creating miniature landslides. Their traps must be effective because antlions are abundant insects and they have been around for millions of years. During its larval life of a year or more, the antlion builds hundreds of pitfall traps and catches hundreds of insects. And yet, when the timing is right, it instinctively knows exactly how to construct a protective cocoon beneath the sand where it will gradually transform into a pupa and ultimately into a winged adult. The sand cocoon, with its glistening crystals of quartz, mica and feldspar, is truly a work of art.

The antlion is just one of many obscure creatures living in the southwestern United States. They are a marvelous little insect to observe in the wild, and they make a fascinating demonstration for a sand terrarium in the classroom. Although they live in a very competitive world, often within disturbed, urbanized areas, they are masters of survival under adverse conditions. If their little crater-like pitfall traps in the sand are obliterated by the wind, rain, animals, or popular two, three or four-wheel vehicles, they just rebuild it and calmly wait for their next prey. In fact, it is this ingenuity and perseverance that undoubtedly explains their survival through countless centuries.

The following information about antlions is summarized from Thomas Eisner's fascinating book For Love of Insects (2003). Ants of the large subfamily Formicinae have a very successful method of defence that involves the discharge of concentrated formic acid. They all possess a formic acid gland, a larger sac in the posterior part of their abdomen. With a concentration exceeding 50 percent, the formic acid is an effective irritant to their predators. When ants are pierced by the sharp jaws of the antlion, they are injected with salivary fluids. The soft insides of the ant are liquified by digestive enzymes and the ensuing "soup" is sucked up by the antlion through its hollow jaws. Studies conducted by Eisner and his colleagues indicate that antlions carefully avoid piercing the contents of the formic acid sac. In addition, the ants only discharge formic acid when they are biting their prey or predator. Ants are unable to bite the antlion's body because of its position beneath the sand. So even though antlions are repelled by formic acid, they avoid this irritant when catching and feeding on ants.


The Evolution Of Microsoft Compared
With The Natural Selection Of Antlions

For dubious believers, it is difficult to imagine how the antlion larva could have ever developed the innate ability to construct its ingenious pitfall trap. Even more perplexing, how could this innate programming have evolved from random genetic mutations. Whether the programming evolved gradually in stages, or whether it evolved more rapidly in a major genetic change, is difficult to say with absolute certainty. The bottom line here is that we are dealing with intense competition among vast numbers and varieties of insects over countless millions of years. There were probably many genetic changes that resulted is dismal failures, but larvae programmed with the peculiar behavior for tossing out sand to make funnel-like craters apparently survived BETTER and successfully passed on MORE of their genes to future generations. By comparison, the success of the Kingdom of Microsoft is based on clever and timely strategic decisions made by brilliant top level executives (such as Bill Gates), capitalizing on the strengths and weaknesses of competing software companies, and just plain luck. It is doubtful that random changes figure into the Microsoft success formula. Like evolving living systems, the success of Microsoft is based on intense competition, excellent (better?) products, elimination of competitors, and survival of the fittest. However, the selective forces on Microsoft are working on carefully orchestrated products designed by a creative minds, not on random changes that just happen to lead to a successful business.

The entire staff at Wayne's Word unanimously concludes that the evolution of Microsoft is NOT a good example of evolution by means of Darwinian natural selection. We also believe that the success of Microsoft has absolutely nothing to do with the amazing antlion scenario discussed in this article.

References:

  1. Eisner, T. 2003. For Love of Insects. Harvard University Press, Cambridge, Massachusetts.

  2. Evans, A.V. 2007. Field Guide to Insects and Spiders of North America. Sterling Publishing Co., Inc. New York, New York.

  3. Hogue, C.L. 1993. Insects of the Los Angeles Basin. Natural History Museum of Los Angeles County.

  4. Hutchins, R.E. 1966. Insects. Prentice-Hall, New York.


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