Butterfly Love

Spring is in the air here in Tucson, Arizona, and apparently the thoughts of butterflies are turning to love. I wrote the following for the Valentine’s Day issue of the volunteer newsletter at the Tucson Botanical Gardens, but cruising the grounds this past week I saw plenty of native butterflies in “action,” so to speak, like this pair of Giant Swallowtails outside our administration building.

Visitors to Butterfly Magic will never fail to notice when two butterflies are conjoined, and will ask you what is going on with that. Well, ok, so the butterflies themselves are not shy when it comes to courtship and sex. They have a limited amount of time to find acceptable mates and produce the next generation.

Among the more dazzling behaviors butterflies engage in is courtship. There are great differences in courtship behavior from family to family, and even species to species, but you can easily recognize certain postures and flight styles as romance-intended. Females may advertise their availability by perching with wings open and abdomen slightly raised. She may also “call” males by releasing a special chemical called a pheromone. Males detect the wind-wafted scent with their antennae and then quickly recognize her visually. Male giant silkmoths like the African Moon Moth and the Forbes Moth can home in on a female from up to a mile away (maybe even longer) by following her pheromone trail.

Male butterflies of many species have pheromones, too, designed to communicate individual fitness to a potential mate. Once he locates a female the male must convince her he is a worthy investment. He may do this by following her in flight until she lands, then hovering over her and showering her with his own “cologne” emitted from special scent patches on his wings, or from “hair pencils,” glands that he extrudes from the rear of his abdomen. Males of some Heliconiinae (longwings) go a step further and sprinkle an anti-aphrodisiac once mating has occurred. This discourages other males from usurping his genetic investment in that particular female’s offspring.

Should the female be disinterested in a suitor, she changes her posture, pointing her abdomen nearly straight up and essentially “mooning” the male.

The pursuit and hovering displays are characteristic of the male Priamus Birdwings and the Heliconius longwings. Morphos are less elegant. Males will land next to perched females and aggressively “nudge” them into compliance. Watch as a male bends his abdomen forward in an attempt to copulate.

Compatible males and females may eventually couple, tail-to-tail, facing in opposite directions. Males might even hang limply from the female as she remains perched. Occasionally the pair will even take flight, one of them carrying the other. Butterflies can remain coupled for as little as a few minutes to several hours. We had one pair of Priamus Birdwings (shown above) engaged for so long that the male actually perished while still connected to the female. While lengthy mating leaves both butterflies vulnerable to predation, it also prevents other males from mating with a given female, increasing the odds that the male will see some of his genes represented in the next generation produced by that female. Nature is full of such trade-offs.

Butterflies have the same sex organs as other animals, but they go by different names. The male penis is called an aedeagus (ee-dee-AY-gus). The shape and configuration of the aedeagus varies from species to species, largely preventing hybridization between different species. He also has claspers, the external genitalia that hold the couple together during sex. The female has a vagina (the “bursa copulatrix”), but also has a “receptaculum seminis” or “spermatheca,” a sac that stores sperm. Her eggs will not be fertilized until she lays them.

Whew! I managed to get through all that without even talking about contraception and the Butterfly Vatican.

A pair of Texan Crescents is shown above.

Wasp Wednesday: Agathidinae

”Why the long face?” is a question that could be asked of some members of the subfamily Agathidinae in the family Braconidae. The narrow, extended face is characteristic of the genera Agathis, Bassus, and Cremnops, helping to make them recognizable in the field (though they can still be confused with other kinds of Ichneumonoidea).

These are also often colorful wasps, clad in red and black, with black or smoky wings. At 6-10 mm, they are small in size, but larger than many other kinds of braconid wasps. They are also frequent visitors to flowers, which sets them apart from most others in the family Braconidae. They make use of those elongated mouthparts to probe for nectar.

The female wasps also hunt caterpillars to lay their eggs in. Typically, one wasp larva develops as an internal parasite of a host caterpillar, though some species are gregarious, several larvae sharing the same caterpillar host. A select few species are occasionally employed as biological control agents. There are 99 species in the subfamily in North America, according to this recent synoptic review of the Agathidinae.

Lots of confusion remains concerning the classification. M. J. Sharkey recently resurrected the genus Lytopylus, splitting it from Bassus, for example. The generic identity of the specimen imaged here, just a few days ago, remains a mystery since I did not collect the specimen, and do not have access to an electron microscope, about the only tool able to render enough detail to facilitate identification at the genus level.

Still, it can be satisfying being able to recognize these insects as braconids in the first place. Most braconids are easily confused with ichneumon wasps of similar size and appearance.

Sources: Marsh, Paul M., Scott R. Shaw, and Robert A. Wharton. 1987. “An Identification Manual for the North American Genera of the Family Braconidae.” Memoirs of the Entomological Society of Washington. Number 13: 98 pp.
Sharkey, M. J. 2004. “Synopsis of the Agathidinae of America North of Mexico,” Proceedings of the Russian Entomological Society. St. Petersburg. Vol. 75(1): 134-152.

Wasp Wednesday: Wasp/Not Wasp

Previously on “Wasp Wednesday” we featured keyhole wasps of the genus Trypoxylon. Like most stinging insects, these wasps are models for other harmless insects that escape predators by pretending to be something more dangerous than they are: a sheep in wolf’s clothing if you will. Meet Pseudodoros clavatus, a near perfect imposter of a keyhole wasp.

A biologist friend once cautioned me on jumping to conclusions about which insects serve as models for various mimics, but just look at this fly. The elongated abdomen even has pale markings to make it appear narrower than it actually is. At 7-12 millimeters long, it is also similar in size to many Trypoxylon wasps. Besides looking like the wasp, the fly even behaves like it. Male keyhole wasps often hover in front of vertical objects, and the fly hovers just as well, if not better than, the wasp.

Ok, so how do you tell them apart, anyway? Above is the wasp. Note that it has two pairs of wings (though connected to each other they are usually still discernible as separate). The antennae of the wasp are thick and relatively long. They eyes are large, but do not take up the entire head or face of the wasp. Now look at the fly below. It has only one pair of wings. The antennae are so short they are scarcely visible. The eyes cover most of the head of the fly, and are not notched on the inner margin like those of the wasp.

Pseudodoros clavatus is a member of the family Syrphidae, collectively known as “flower flies” (“hover flies” in Europe). They are frequent, abundant visitors to flowers of all kinds. While they may be insignificant pollinators, they play their part in perpetuating wildflowers. Their good deeds extend to the larval stage as well. The maggots of Pseudodoros are voracious predators of aphids.

Seeing one of these slug-like larvae on your rose bush might lead you to think that it is also eating the plant, but watch one closely and you will see it methodically slaying aphids, seizing the tiny sap-suckers and hoisting them off the stem. The fly larva then sucks the hapless pest dry and discards the empty husk of its exoskeleton.

Look for this species from coast to coast in the United States and southern Canada. It may be confused with the similar genera Baccha and Ocyptamus in some parts of its range, but Ocyptamus species have at least faint dark markings on the wings (if only a bold leading edge to the wing). Baccha is more difficult to distinguish, but at least one species has a distinctly banded abdomen. The image below is still a Pseudodoros.

Wasp Wednesday: Trypoxylon

Last week I reported on the “Keyhole Wasp,” Pachodynerus nasidens, but mentioned that there are other kinds of wasps that also go by that common name. Meet the members of the genus Trypoxylon in the family Crabronidae. Most species of these spider-hunting wasps nest in pre-existing cavities, such as old nail holes. They are common, but usually go unnoticed by the casual naturalist.

Trypoxylon wasps can be separated from other wasps by a couple of distinctive characters. The abdomen is long, slender, and clavate (club-shaped, the tip of the abdomen being decidedly blunt). There is only one submarginal cell in the forewing. Lastly, the compound eyes are emarginate, meaning they are obviously “notched” in the middle, on the inner margin.

The genus is divided into two subgenera, the small species in the subgenus Trypoxylon, while medium-sized and large species are in the subgenus Trypargilum. Both groups are collectively distributed over most of the North American continent. The only ones I have managed to image in the wild so far are species of Trypargilum. Despite their larger size, they defy identification to species without collection of actual specimens. However, their shared life cycles are similar enough to generalize.

Adult females are solitary, and seek natural cavities in which to nest. This typically means hollow twigs, old beetle borings in dead, standing trees, vacated insect galls, and abandoned nests of mud dauber wasps. Those natural tunnels are progressively partitioned into individual cells along their length as the wasp first provisions the deepest one, then moves forward toward the entrance.

Interestingly, the males often participate in nest maintenance, at the least by actively guarding the nest entrance while the female is away hunting prey or harvesting mud for one of the partitions. This is extremely valuable because the nest is besieged by parasitic insects that would find easy pickings were it not for the male wasp’s devotion to duty. Males do not sting, but they physically block entry of parasites and behave aggressively toward those enemies. Observers have also noted that in some species of Trypoxylon the males may take prey brought in by the female and place it in the cell while she resumes hunting. Males can also assist in cleaning out the cavity before nesting begins, and aiding in the building of partitions and the mud plug closing the finished nest. Naturalist Phil Rau called those species demonstrating couples teamwork “Patriarchate wasps.”

The two species found commonly here in Tucson, T. californicum and T. clavatum clavatum, average from 1-5 cells per nest. Spiders are the prey of these wasps, and mostly immature spiders at that. Some species hunt mostly web-building spiders, while other species seek free-ranging spiders, but there always seems to be at least a slight degree of overlap in prey selection. Jumping spiders, crab spiders, running spiders, lynx spiders, and sac spiders are recorded prey for T. californicum. The same general prey base applies to T. c. clavatum, but also add small wolf spiders and a good selection of orb weavers. Anywhere from 5-20 spiders are stocked in each cell, sometimes more, occasionally less (3 is the recorded minimum, 36 the maximum).

One egg is laid in each cell, and the larva that hatches consumes the cache of paralyzed arachnids. When mature, the larvae spins a cocoon. Apparently, the composition and architecture of the cocoon is species-specific, with varying amounts of silk, saliva, and soil going into the matrix. Inside the cocoon the larva pupates, and an adult wasp eventually emerges.

I would be remiss if I failed to mention that there is one species in North America that breaks all the rules. That would be the “Pipe Organ Mud Dauber,” Trypoxylon politum. This is a very large wasp for the genus, glossy black with white “ankles” on the hind legs, and it constructs large, linear mud nests that resemble the pipes on an old-fashioned organ. This wasp ranges over most of the U.S. east of the Great Plains. You can learn more about it in this excellent archived entry from ”This Week at Hilton Pond” and at Bugguide.net.

You can create habitat for the cavity-nesters by simply drilling holes in a block of wood and hanging it up under an eave on your home, shed, or barn. One pair of researchers here in Tucson used a ¼ inch bit drilled to a depth of 77 millimeters, and a 3/16th inch bit drilled to 124 millimeters, with good success. A good primer on how to build such “trap nests” can be found here. Give it a try!

Sources: Matthews, R.W. and J.R. 1968. “A note on Trypargilum arizonense in trap nests from Arizona, with a review of prey preferences and cocoon structure in the genus.” Psyche 75: 285-293 (available online as a PDF. T. arizonense is an outdated name for T. californicum).
Bohart, R.M. and A.S. Menke. 1976. Sphecid Wasps of the World. Berkeley: University of California Press. 695 pp.