Friday, April 17, 2015

How Insects Sing

Observing the insect world requires one to use not just their vision, but their hearing, too. Many insects use sound to communicate with each other, or to ward-off enemies by startling them with a sudden, audible noise. How do insects make such a racket, anyway?

Male field cricket, Gryllus sp., Arizona
Stridulation

Most insects produce sound by rubbing one body part against another, a phenomenon known as "stridulation." The most accomplished stridulators are, of course, the members of the order Orthoptera. Most male grasshoppers, katydids, and crickets have modifications of the body that allow them to create, amplify, and broadcast auditory signals to attract mates and/or ward-off competing males.

Male larger meadow katydid, Orchelimum sp., singing in Nebraska

Short-horned grasshoppers in the family Acrididae rub special pegs on the inside surface of the hind femur ("thigh") against raised veins on the front wings while the insects are at rest. Slant-faced grasshoppers (subfamily Gomphicerinae) are especially accomplished at this and their soft zip-zip-zip songs are easily heard during daylight hours. Below is a video of a male Psoloessa grasshopper stridulating vigorously. Unfortunately, the incessant prairie winds on the eastern Colorado plains overwhelm the 'hopper's song.

Male Psoloessa sp. grasshopper stridulating

Band-winged grasshoppers (subfamily Oedipodinae) also stridulate, but not as loudly. You are more apt to see two males sizing each other up instead of a male courting a female. In the video below, two male Carolina Grasshoppers stridulate aggressively, side-by-side, until the loser leaves the stage.

Male Carolina Grasshoppers, Dissosteira carolina, stridulating aggressively

Katydids and crickets are the most accomplished of all stridulating insects, in part because the wings of the male insect are highly modified to produce sound; and the insect often stations itself in a circumstance that enhances the projection of sound, such as the mouth of a burrow, or between leaves.

Male tree cricket, Oecanthus sp., singing, Colorado
Male tree cricket at rest

Most male katydids are "left-handed," meaning that the left forewing overlaps the right forewingwing. Contrastingly, most male crickets are "right-handed." In both cases, the edge of one wing is equipped with a "file" of fine teeth, while the other wing has a bladelike "scraper" that is drawn rapidly over the file to create the song.

Singing male field cricket, Gryllus sp., Colorado

The songs we hear most often are "calling songs" designed to attract females; but, crickets in particular produce two other types of songs: a "rivalry song" that is directed at another male during and/or after a confrontation, and a soft "courtship song" to entice a female into mating.

Male Drumming Katydid, Meconema thalassinum, Ohio
Percussion

A surprising variety of insects, from treehoppers to stoneflies, to beetles actually smack body parts against a substrate such as a leaf or twig to generate vibrations that are received by potential mates. Sometimes these body-slamming signals are also audible. The Drumming Katydid, Meconema thalassinum, and "wing-tapper" cicadas in the genus Platypedia do not produce sound the way most other members of their clan do. Instead, they strike a blow against a branch, leaf, or twig to call to the opposite sex. This is still audible to us, too.

Wing-tapper cicada, Platypedia putnami, Colorado

Males of most cicada species have a pair of built-in percussion instruments. Large pits take up most of the volume of the abdomen and are covered by "lids" called opercula (singular: operculum). Inside each chamber, a strong muscle pulls and releases another organ called a tymbal. The tymbal buckles under the muscle's tug, generating a noise, then snaps back when the muscle relaxes, making another noise. The muscle twitches at such a high rate that we hear one continuous sound, and a very loud one at that.

Underside of male Tibicen sp. cicada showing operculum
Hearing

How do the insects hear each other? Insects do not have ears, per se, but they do have auditory receptors called typmana located in peculiar places. Short-horned grasshoppers have an opening on each side of the abdomen, near the base. Cidadas likewise have their typana located in the front section of the abdomen. Katydids and crickets have a slit located on the front surface of the tibia ("shin") on each front leg. The tympana is usually represented as a thin, oval membrane located inside the "ear" opening.

Greater Anglewing Katydid, Microcentrum rhombifolium, showing "ear"
Resources

Many singing insects can be identified reliably only by differences in their songs, so it helps to familiarize yourself with their calls. Thankfully, a variety of resources are available to do just that. The "Singing Insects of North America" website is particularly useful. Examples of songs are available as audio files for most species.

Male conehead katydid, Neoconocephalus ensiger, singing, Massachusetts

Lisa Rainsong's superb scientific blog "Listening in Nature" covers bird songs, frog choruses, and other sounds of nature as well as insect songs.

Several books exist that address insect songs, but my two favorites are The Songs of Insects by Lang Elliott and Wil Hershberger (2007, Houghton Mifflin Company), and Crickets and Katydids, Concerts and Solos, by Vincent G. Dethier (1992, Harvard University Press).

You should be hearing field crickets any time now, if you aren't already, along with the daytime calls of some acridid grasshoppers. Enjoy the symphony.

Saturday, April 11, 2015

Anteater Scarab Beetles

Beetles have truly bizarre lifestyles in general, but one of the most astonishing is the life cycle of "anteater scarab" beetles in the genus Cremastocheilus. These are rather small, non-descript insects that are easily overlooked. What a pity. I guarantee you will want to find one yourself after reading this story.

This is the time of year when you are most likely to encounter these beetles. They are dispersing themselves to find new hosts. They fly very well, and fast, but crash land in places frequented by scouting ants during the day.

There are roughly 35 species of Cremastocheilus currently recognized in North America north of Mexico, but the genus is being revised. Most are dull black, about 10 millimeters in body length, flattened, and very angular. The exoskeleton is exceedingly dense, often with pits or wrinkles, or both. These are heavily armored insects, and for good reason.

Anteater scarab beetles are just that: they feed as adults on the larvae and sometimes the pupae of ants. The host ant species are mostly mound-builders in the genus Formica, but other hosts include honeypot ants (Myrmecocystus), cornfield ants (Lasius), and carpenter ants (Camponotus). Harvester ants (Pogonomyrmex and Veromessor), and spine-waisted ants (Aphaenogaster), are frequent hosts for anteater scarabs in the southwest U.S. The heaps of vegetation created by some Formica species, known as "thatching ants," provide the protection and food resources needed for Cremastocheilus beetles to complete their life cycle.

Cremastocheilus being dragged by Formica ant

How do the beetles gain entry to an ant nest protected by hundreds, if not thousands, of short-tempered worker ants? It appears that the beetles "play dead" in areas heavily trafficked by roaming worker ants. The death-feigning posture of anteater scarabs is highly convincing: motionless, with legs outstretched. Thatching ants are scavengers and sometimes predators, so any morsel of protein is quickly snatched up and taken back to the nest.

The antennae of Cremastocheilus are normally protected in grooves under the head, but here they are seen on an active specimen

The female beetles lay their eggs singly in the soil around the periphery of the nest mound, and the larvae feed on the decaying vegetation that the ants have more or less composted.

Larvae are pretty much ignored by the ants, though the grubs are capable of fighting back by striking attacking ants with their jaws, and secreting repulsive liquids from their mandibles and anal glands.

The beetle grubs progress through three instars, an instar being the period between molts, before pupating in oval chambers they create from soil particles. The cycle from egg to adult takes about four months, adult beetles emerging from their pupal cells in late summer or early fall.

The newly-emerged beetles make their way slowly to the center of the ant nest, accumulating the colony scent along the way and presumably rendering themselves incognito. The adult ants are also largely dormant during the winter. Come spring, the beetles begin feeding on ant larvae.

Close-up of mentum on underside of the beetle's head © Jeff Gruber and University of Wisconsin (Madison) Entomology Dept.

The beetles have unusual modifications to their mouthparts. The mentum is enlarged and shield-like, covering the other mouthparts from below. The maxillae are modified into sharp appendages that pierce the soft cuticle of ant larvae and pupae. The beetle's mandibles are much reduced and function only to help transport liquid food to the mouth.

Cremastocheilus beetles have special brushes of hairs on the hind angle of the pronotum (top of thorax) where it meets the "shoulders" of the elytra (wing covers). These brushes are called trichomes. The hairs are attached to glands that secrete a liquid substance that was originally interpreted as an attractant, or at least a pacifier, to the ants. This is apparently not the case, and the trichomes remain something of a mystery.

Anteater scarabs reach their greatest diversity in the desert southwest, but they can turn up almost anywhere. Do a double-take when you expose an ant colony under a stone or board. There may be a Cremastocheilus or two among the ants. Excavated ant colonies usually yield twenty or thirty beetles or their larvae, but some nests may host up to 200.

Sources: Alpert, Gary D. and Paul O. Ritcher. 1975. "Notes on the Life Cycle and Myrmecophilous Adaptations of Cremastocheilus armatus (Coleoptera: Scarabaeidae)," Psyche, 82 (3-4): 283-291.
Hölldobler, Bert and Edward O. Wilson. 1990. The Ants. Cambridge, Massachusetts: Belknap Press (Harvard University Press). 732 pp.
Mynhardt, Glené and John W. Wenzel. 2010. "Phylogenetic analysis of the myrmecophilous Cremastocheilus Knoch (Coleoptera, Scarabaeidae, Cetoniinae), based on external adult morphology," ZooKeys 34: 129-140.
Ratcliffe, Brett C. 1991. "The Scarab Beetles of Nebraska," Bull. Univ. Nebr. State Mus., vol. 12: 333 pp.

Monday, April 6, 2015

Springtime Tiger Beetles

Early spring along the Front Range of Colorado is deceptive. Few flowers are in bloom by mid-March, and there is little color to indicate that life has changed since winter's onset. Tiger beetles to the rescue! So far this spring, local bug-watchers have seen and photographed nine species of these exquisite animals.

Oblique-lined Tiger Beetle, Cicindela tranquebarica

Tiger beetles were once placed in their own family, Cicindelidae, but are now considered a subfamily (Cicindelinae) of the ground beetles, family Carabidae. Their taxonomic "demotion" has not diminished enthusiasm for these charismatic predators, a favorite of entomologists and increasingly popular among naturalists in general.

Twelve-spotted Tiger Beetle, Cicindela duodecimguttata
Habitat

The common tiger beetles of the genus Cicindela prefer open, sandy habitats such as beaches, dunes, and barren paths through prairies and forests. Some are agile enough to negotiate obstacles on rocky shores and riverbanks, too. It is frequently a matter of luck finding them. Perfectly appropriate habitats may be devoid of tiger beetles, while other locations are chalk full.

Splendid Tiger Beetle, Cicindela splendida
Appearance

Cicindela tiger beetles average 11-14 millimeters in body length. Most are brown with ornate ivory markings, but a few are metallic green or red, or both. Most are decorated with white hairs that help insulate them from the hot sun. They have large eyes, long legs, and enormous mandibles.

Western Tiger Beetle, Cicindela oregona
Feeding

Most tiger beetles you see will be running away from you, but sit and watch and eventually they become accustomed to your presence and resume normal behavior, like hunting. Tiger beetles pursue small invertebrates, including ants. The beetles have excellent vision and quickly detect the movement of a potential meal. They run after it, but may stop before they reach their target. Tiger beetles are so fast they can actually outrun their ability to focus. So, they stop, re-focus, and dash off again. Once they overtake their prey, those menacing jaws quickly reduce the victim to shreds.

Blowout Tiger Beetle finishing lunch
Mating

Another favorite activity of tiger beetles is making more tiger beetles. You will often see one beetle atop another. The one on top is the male. The pronotum (top of thorax) of the female of each species has notches and dents that will only accept the toothed mandibles of the corresponding male. This stops interbreeding with the wrong species, but allows the right male to "get a grip" on the object of his affection.

Green Claybank Tiger Beetles, Cicindela denverensis

Competition for the girls is keen, and sometimes a male will be late to the party. The result can be a comedic "conga line" of stacked cicindelids.

Blowout Tiger Beetle, Cicindela lengi
Thermoregulation

Even in the relative cool spring and fall months, tiger beetles are in danger of overheating on the scorching sand. Consequently, they may seek shade under low-growing plants. They also use their long legs to elevate themselves high off the ground. Being on "tip-toe" makes them no less wary and agile.

Festive Tiger Beetle, Cicindela scutellaris
Colors

The metallic nature of tiger beetles is structural, rather than the result of pigmentation. Layers in the exoskeleton reflect various wavelengths of light. Despite their colorful appearance, the beetles are still surprisingly cryptic. When disturbed, tiger beetles can fly, then alight facing the direction of the threat, reducing their profile. Sometimes, they hunker down in a depression in the sand, becoming even more inconspicuous. Ivory patterns on the elytra (wing covers) break up the outline of the beetle even more.

Bronzed Tiger Beetle, Cicindela repanda
Seasonality

The tiger beetle species present as adults now are spring-fall species that spend the summer as larvae and pupae in tunnels in the soil. The larvae are ambush hunters that each live in a vertical shaft of their own making. They present their flat heads at the entrance to the burrow, flush with the opening. Any small invertebrate that passes by risks being seized by the larva.

The summer months see a different set of species that share the same habitat. So, visiting the same site at different times of the year can yield an entirely different fauna.

Purple Tiger Beetle, Cicindela purpurea, green form
Purple Tiger Beetle, Cicindela purpurea, blue form
Species

The species we have seen here on the plains and east slope of the Rocky Mountains this year include: Oblique-lined Tiger Beetle, Cicindela tranquebarica; Bronzed Tiger Beetle, Cicindela repanda; Twelve-spotted Tiger Beetle, Cicindela duodecimguttata; Western Tiger Beetle, Cicindela oregona; Green Claybank Tiger Beetle, Cicindela denverensis; Purple Tiger Beetle (aka "Cowpath Tiger Beetle"), Cicindela purpurea; Blowout Tiger Beetle, Cicindela lengi; Festive Tiger Beetle, Cicindela scutellaris; Splendid Tiger Beetle, Cicindela splendida. Each species can be highly variable in markings, with enough overlap with other species to make matters confusing to the novice observer. Pay attention to habitat, behavior, and size.

Festive Tiger Beetle, Cicindela scutellaris
Enjoy!

Mostly, simply enjoy watching these spirited insects as they go about their lives. At the least it is a rewarding diversion, but it can lead to new discoveries, too. Our understanding of even the distribution of tiger beetle species is poor; and because they are found in habitats vulnerable to human development, some species are threatened or endangered.

Sources: Acorn, John. 2001. Tiger Beetles of Alberta. Edmonton: University of Alberta Press. 120 pp.
Schmidt, Justin P. 2002. "Cicindelidae of Colorado," Colorado State University.

Friday, April 3, 2015

Courting Robber Flies

It must be spring. Love is in the air, on twigs, on the ground....at least if you are watching insects. They are courting and mating everywhere. My last couple of outings afield on the plains here in eastern Colorado have given me the opportunity to witness some insect romance up close; and I remembered to use the video function on my camera, too.

Male Ablautus(?) robber fly with leafhopper prey

One particularly abundant insect in early spring is a small robber fly, family Asilidae, possibly in the genus Ablautus. Adults of this dipteran measure only about 6-7 millimeters, and they are sand-colored, so rather difficult to see. They prey on insects smaller than they are. The one in the image above has killed a leafhopper.

I discovered that the males are slightly smaller and more slender than females, with bright white hairs on their faces. Females have gold or ochre-colored hairs on their faces.

Female Ablautus(?) robber fly

Males are very energetic, and once engaged in courting a female, they are not easily frightened away. So, once I discerned there was a pair of the flies on the ground in front of me, I stopped and watched. This is the amazing result. The male alights near the stationary female, then flies and alights again, sometimes sidling into a different position. Eventually, he waves his front legs in front of her and rocks up and down. It is quite a display.

This next video is of a different pair of flies, perhaps a little farther along in the process, because it has an, uh...."happy ending." The male is the one with the smile on his face.

Many flies have elaborate courtship rituals, but asilids (robber flies) have some of the fanciest. I was lucky enough to also capture video of the aerial display of Heteropogon macerinus in Ohio a couple of years ago.

Robber fly "wedding photo"

I urge you to find your own examples of "bug love," and document them with your camera, even your smartphone. There are plenty of places to share the results online. Maybe your video will even go viral on YouTube. In any event, we stand to learn a good deal more about insect behavior through the efforts of curious citizen scientists.