Saturday, April 11, 2015

Arthropods as builders

Construction projects of the arthropod world: They may be shelters, nurseries, traps for prey, or 'greenhouses' for farming, storage facilities, even floating devices - Arthropods can produce the most amazingly complex and functional structures. To complete those, they may work alone or together with hive members, building with hard labor or by chemically stimulating a host organism to do the work. Structures may be built for duration or destroyed and rebuilt every 24 hours. They may be built for a special event in a life cycle of the insect or for everyday needs.

Many ants build nest mounds. In Germany I often saw huge piles of material containing chambers that contained different stages of larval and pupal development - nurseries that would be moved around within by workers when the temperature changed (Rote Waldameise (Formica rufa)). In other species, piles of soil seem to be just material that is moved out from underground chambers, or refuse that is piled around the entrance of a nest. But the structures that our local leafcutter ant Acromyrmex versicolor builds seem much too well designed to be just piles of refuse. They are perfectly round cones with a crater-like opening when they are new. They erode later, but are often restored. Most colonies have several of these structures. These ants are fungus growers. Their activity level varies very much with season and weather. I assume that they have ways to regulate the growth rate of their fungus gardens in correlation to their own activity level, and I believe that those cones are actually chimneys that control the ventilation of those underground gardens. Either they need more ventilation during high growing seasons, or the ants can even promote faster growth by building higher (or more) chimneys.

 Termites in Africa and Australia build long lasting tall structures, but our Desert Encrusters only work on their adobe structures when the weather is just right after the monsoon rains. They completely cloak dead wood with a layer of mud under which they are protected from the sun while they are slowly rasping away the top layer of dead plant material. Even scar tissue on saguaros gets this treatment. Living tissues are not harmed. Their relatives, the feared subterranean termites, are constructing tell-tale mud tubes to get safely to their deconstruction sites - a thing to be aware of. Some of our termites also build mud turrets as launching platforms for their alates. Every summer, those winged, sexual specimens are waiting within the colony, ready to swarm under just the right weather conditions. When those are perfect, the workers literally push the alates out of those turrets to take flight.

Other mud tubes are the nests of Organ Pipe Mud Daubers. These sphecid Wasps  catch and paralyze spiders as provision for eggs that are placed in rows of chambers covered in mud. These are from Sonora Mexico and were located in a protected spot under an overhanging rock formation.  

Anthidiellum sp., Resin bee, nests, Megachilidae
 Flowers need to seduce insects to visit and pollinate, and the reward is not always nectar. Some flowers offer building materials to their pollinators, for example resin to some species in the family Megachilidae. Female Resin Bees attach single cell nurseries to twigs, some even work little pebbles into the structure.    

Wasps and Hornets developed paper long before even the Chinese knew about it. Chewing cellulose and lignin rich plant material, the insects produce an extremely light and durable building material. Series of hexagonal cells can be tightly packed, forming  structurally sound and expandable mass nurseries and storage chambers. In many cases a thin stylus supports hundreds of cells. The wax honey combs of bees are much heavier, so their builders usually fit them into preexisting niches and cavities.  

 Here in AZ a little Eurasian weevil Coniatus spendidulus is taking on the rapidly spreading tamarisk bush, also from Eurasia. It is not clear whether it was released on purpose or not and it seems to be far less destructive to this botanical pest than some leaf beetles that were introduced. But the tiny guys are interesting in that they have larvae and pupae on the plant surface and do not go into the ground to metamorphose. Instead the last instar larva builds this cage-like cocoon and pupates inside. The whole fragile thing is barely 2 mm long, so I do not know how it protects the pupa, except from falling off the twig.

Leaf-rolling Weevils (family Attelabidae) lay their eggs on oak or Hazel, Alder etc. leaves. The female then cuts the leaf and rolls it very tightly, preserving the mid-rib. The larva lives and develops in the 'nidus' that is formed. But when I collected some, a different, related weevil, the Thief Weevil, hatched. This beetle crawl carefully into the fresh nidus and replace the original egg with her own. So the brown weevil Himatolabus pubescens is the constructor and the blue one, Pterocolus ovatus. is the thief. Did I mention that the thief is a cousin in the same family Attelabidae?
Madera Canyon, April when the oak leaves are fresh

 Many caterpillars use their silk glands to spin protective tubes or pull leaves together as a shelter. But the communal nests of some Tent Caterpillars (Malacosoma) stand out for their in size and strength. In the image you see that the caterpillars enlarged  the nest repeatedly as they grew from tiny hatchlings to 2 in caterpillars. They spend nights and periods of cold weather inside. The metabolism of the caterpillars and their accumulated droppings produce heat. The toughness of the silk walls and shed urticating hairs keep many predators out. But some warblers learn to pick up any caterpillars that venture to the outside. And they need to come out to feed. When they do so, they leave silk strands along their path to guide them back to safety later. Scouts seem to lead the way. The caterpillars can defoliate young wild cherry trees, aspen or willows, but usually the trees will not be harmed permanently. I have collected tent caterpillars for a project and 80% yielded parasitic tachinid flies instead of moths.

:The tiger moth Euchaetes elegans lays her eggs on host milkweeds and covers them with the bright white scales from her belly. I presume this helps protect them from parasites, predation and perhaps the weather, but does nothing to hide them. The mature larva also incorporates hairs into its cocoon. The larva, however, is covered in black, orange and a few white hairs so the cocoon looks nothing like the egg mass. Here in Tucson you can find all stages of this insect during the monsoons, when climbing milkweeds are abundant. Images and text contributed by Randy Hardy

We have moths but also beetles that are called 'case bearer' because their larvae build little covers that they carry around. Our Warty Leaf Beetles look like caterpillar droppings as adults, but their larvae carry these little magician hats. The material is probably feces as in the protective shields of many leaf beetle larvae, plus plant fibers it seems.

Snailcase Bagworm (Apterona helicoidella) and Oiketicus sp.
 Psychidae (Bagworm Moths) Larvae (bagworms) construct snailcase or spindle-shaped bags covered with pieces of twigs, leaves, etc., and remain in them enlarging the bags as they grow -- until they pupate (also in the bag). Adult females remain in the bag even then, emitting pheromones which attract adult males to mate with them. The wingless female then lays her eggs in the protection of the bag.

Plant gall inducers:
Insects of several different orders like hymenoptera and diptera and hemiptera have developed the ability to induce host plants to generate extra tissues especially for their use: plant galls. Unlike tumors, they grow into a very specific shape in a specific place. On the same oak you can find stem and leaf galls, round ones that are smooth and woody and others patterned like marbles or fuzzy like little animals. Most contain a developing insect but some, like the stem galls on Cottonwood, contain a whole colony of aphids.

My friend Jim Zimmermann devoted decades of his retirement to the collection of oak galls in Arizona. He cataloged the host plant and the insects that hatched - often not just the insect that induced the gall to grow (mostly wasps), but also parasites that also used the gall tissue and others that used the  larva of the gall wasp as their host. Secondary parasites of those made the system even more complex. He donated the whole incredible collection to the U of A insect collection and still spends many hours every week working on it and other hymenoptera groups.

Froth produced by a spittlebug nymph, lower right.
Not a yet a permanent,  hard construction (but some relatives of spittlebugs build something similar that calcifies)...
Spittlebug nymphs, like all hemipterans, suck plant juices. Like other plant suckers, they take up much more sap than they need and have to get rid of the excess. While aphids just excrete honey dew and sharp shooters eliminate it in sudden bursts (shots) Spittlebug larvae  produce a blob of foam. It  protects the tender larva from dehydration and overheating, and hides it from parasites and predators.
I always assumed that the froth is very distasteful, wikipedia says acrid ... until one day I noticed that on certain Seepwillows (Baccharis salcilifolia) every blob of spittle had a guest: Paper wasps, flies, and even a teneb beetle were peacefully licking up fluid under the spittle. But they did not attack the nymph inside.

Most of the constructions mentioned so far have been shelters, but some of the  most amazing achievements of all seem to me the webs of spiders. Structures that often combine the functions of shelter and trap. From trip lines to funnel shapes, to wheel like webs with regular spokes, to huge communal hammocks - many different types of web can be produced. The material is released by spinnerets under the spiders's abdomen. The strands can be made for strength or stickiness or even extra light, to allow the spider to float through the air with it. Pictured is the web of a Cribellate Orb Weavers genus Uloborus. The cribellate threads are electro-static, but not sticky.

Build in sand:
Certain antlion larvae dig pits in loose sand to use as traps to catch their prey. They sit buried to their mandibles at the bottom. When ants walk across the pit they lose traction and slide down into the center where the hunter awaits them. If an ant seems likely to escape, the lion tosses sand at it to cause an avalanche that will bring it down.

Construction was a topic that Robyn Waayers proposed as the weekly theme for our insect group on FB. I was going to simply collect my contributions here, but then it just kept growing .... I still have many more ideas, but this blog is too long already


  1. Very nice summary with regards to this topic.

  2. Hey Margarethe, thanks for sharing. I read it ALL! Nice to see Jim, and to know he donated his collection of oak galls and such to UAIC. A colony of wasps is forming a nest under my patio. Come see it if you like! I'm going to look for warblers next time I'm in sight of a tent caterpillar nest! Thanks for all the insights. CT

  3. I hadn't thought about those well-formed ant cones as possible temperature regulators for underground fungus farms. Neat idea!