Entomology Diary

Monday 13 May 2013

When handling Madagascan giant hissing cockroaches (Gromphadorhina portentosa), you are really holding two arthropod species, not just one.

Indeed, anybody who handles hissing roaches (species in the genera Gromphadorhina and Elliptorhina, Blattodea: Blaberidae) is familiar with tiny mites that live on their bodies. The species of mite is Androlaelaps schaeferi (Arachnida: Acari: Mesostigmata: Laelapidae), formerly called Gromphadorholaelaps schaeferi, the hissing cockroach mite. An adult specimen is shown here on the pronotal shield of a Madagascan giant hissing roach.

Androlaelaps schaeferi occurs only on the large cockroaches of the genera Gromphadorhina and Elliptorhina. These roaches (and hence their associated mites) are endemic to Madagascar, though some hissing roaches, most notably Gromphadorhina portentosa (the Madagascan giant) and Elliptorhina javanica (the Halloween hisser), are now widely bred in the exotic-pet trade of North America.

The relationship between A. schaeferi and its hosts is an example of commensalism rather than true parasitism. The mites roam on the exoskeleton of the host cockroach and eat saliva and organic detritus left over from the host's feeding. For this reason, they are most commonly found on the ventral side of the host roach, near the mouth, though they often wander on the dorsum as well. The mites are not harmful to the roaches and, in fact, seem to confer a slight benefit by keeping them clean and thus helping to prevent fungal growths on the roaches' bodies.

Because they cannot survive off the body of a host, the mites on a roach that dies must transfer quickly to other hosts, but this is often an easy process because of the gregarious nature of hissing cockroaches.

Monday 8 April 2013

Photographed in captivity, a specimen of Theraphosa blondi (Araneae: Theraphosidae), the world's largest tarantula species, prepares to strike an immature giant leaf cockroach, Blaberus giganteus (Blattodea: Blaberidae).

After impaling and envenomating the cockroach with its chelicerae, the tarantula begins the long process of feeding: as it mashes the prey, it soaks the body in a fluid of proteolytic enzymes that liquefy the tissues. After this external predigestion, the spider sucks up the resulting nutritious soup.

Theraphosa blondi inhabits the Amazon rainforest. Like most other tarantulas, it shelters in a burrow and is active only at night, when it emerges to the mouth of its retreat and silently waits for prey to pass close enough for an attack. Blaberus giganteus, incidentally, is one of the world's largest cockroach species and inhabits the tropical forests of Central America and Amazonia.

Sunday 7 April 2013

Photographed in captivity, this is Centruroides gracilis (Scorpiones: Buthidae), the Florida bark scorpion, sometimes called the slender-brown scorpion. The specimen on the left seems to be an adult female; the smaller scorpion on the right looks like a subadult male.

Along with a few different color morphs in this species, the adult size is also quite variable: the outstretched length can be anywhere from 60 mm up to 150 mm.

Centruroides gracilis is indigenous to Central America and some Caribbean islands. Despite the common name "Florida bark scorpion," it remains uncertain whether the species is also indigenous to Florida or whether it was introduced there by people in recent times. Highly adaptable, C. gracilis has readily become established in many of the areas throughout the neotropics where it has been accidentally distributed by human activity, including Mexico and parts of tropical South America. In addition to its current neotropical range, the species has also been introduced in the Canary Islands and parts of West Africa. Like the other bark scorpions (genus Centruroides), C. gracilis does not burrow and uses incidental forms of shelter in its habitat. In the wild, these shelters may include tree crevices and the spaces under stones and logs. In anthropogenic settings, C. gracilis readily takes to the microhabitats offered by structures and is sometimes found indoors. Its sting can be extremely painful, but usually it is not medically serious.

Tuesday 2 April 2013

To me, few animals are more engaging and captivating than the large centipedes of the order Scolopendromorpha.

Kept behind the scenes at the Harvard Museum of Natural History, this is a specimen of Scolopendra polymorpha (Scolopendromorpha: Scolopendridae), commonly called the tiger centipede. It is mature, with 21 body segments after the head and a length of about 12 cm. Scolopendra polymorpha occurs throughout much of western North America, primarily from Texas through New Mexico, Arizona, Nevada, California, and northern Mexico, though tiger centipedes have been recorded as far north as Oregon, Idaho, and southern Montana. The centipedes are nocturnal and shelter by day in soil, leaf-litter, rotting logs, or the spaces underneath stones. While I had the camera out, our specimen showed off its effective burrowing skills.

Like other centipedes, scolopendrids are predators of mainly small invertebrates and subdue their prey with a pair of appendages based just behind the head that have evolved into venom-injecting pincers: in the photo below, the black-tipped weapons curving around the sides of the head.

These are alternately called venom claws, prehensors, or forcipules. As they are not mouthparts, their action is considered to be a pinch rather than a bite. Produced by a gland in the basal portion of each appendage, the venom is believed to have both cytotoxic and neurotoxic properties. Scolopendrid centipedes also wield their venom claws in self-defense. In humans, a venomous pinch from a scolopendrid centipede is often sharply painful and may lead to localized swelling or even a necrotic lesion, but the damage is seldom medically serious, and no deaths from centipede venom have ever been recorded.

Scolopendrid centipedes are swift and vigorous. When disturbed, they scurry in a sudden frenetic, zigzagging race for shelter. If cornered or simply pestered too aggressively, they will whip their heads around and lash at the threat with the venom claws. Prey capture happens at a similarly fast pace: the centipede pursues or pounces on its quarry and then envelops it with its strong limbs before sinking the venom claws into the body. The appendages at the caudal end of a scolopendrid centipede are especially prehensile and are therefore useful for clutching prey—or the strap of my flashlight.

In at least one Venezuelan cave, centipedes of the world's largest species, Scolopendra gigantea (up to 30 cm long), use their caudal appendages to hang from the cave's ceiling, where they catch flying bats. (See Predation by giant centipedes, Scolopendra gigantea, on three species of bats in a Venezuelan cave, J. Molinari et alia, Caribbean Journal of Science, 41(2), 2005, pages 340–346.)

Tuesday 19 March 2013

This is the smallest animal I have ever seriously tried to photograph (please pardon my blurriness). Between 2 and 3 mm long, the insect is Anthrenus verbasci (Coleoptera: Dermestidae), commonly called the varied carpet beetle.

While beetles in general do not much interest me, I am fond of the little Dermestidae. To the unaided eye, they look at first like tiny black specks, barely even recognizable as beetles. A magnified closer look, however, suddenly reveals a marbled mixture of colors.

As with many other beetles in the family Dermestidae (which basically means "skin-eaters"), the larvae of this species inhabit animal nests, burrows, and dens, where they feed on protein-rich organic detritus, including dry skin, hair, feathers, and arthropod remains. Naturally, being rich in these materials (and warm year-round), our own dwellings offer ideal habitats for dermestid larvae, which will also eat food and fabrics derived from plants. Some dermestid species are notorious for damaging preserved animal material in natural-history museums—from insects to stuffed mammals and birds. Indeed, alongside the common name carpet beetles, dermestids are also sometimes called museum beetles. Anthrenus verbasci is the most frequently occurring dermestid species in museums, especially those with collections of preserved arthropods. My fondness for these beetles is certainly not shared by entomological curators who have many thousands of pinned and mounted specimens to protect.

Beyond its famous presence in museums, Anthrenus verbasci is worldwide the most common dermestid species in buildings generally. Perhaps surprisingly, the adult beetles spend much of their time in the open, where they fly about in search of mates and take breaks to feed on flower nectar. After mating, the females seek oviposition sites that offer plenty of food for the larvae. A. verbasci has a special preference for arthropod carrion (which is why it finds natural-history museums so convenient), and in the wild the females often lay eggs in sites offering much arthropodal detritus: bird nests, bat roosts, the nests of bees or wasps, and even spiders' webs. After completing immature growth and development, the larvae pupate into the adult form. The whole life-cycle lasts anywhere between about 250 days and a year.