Mastophora hutchinsoni

Mastophora hutchinsoni
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Chelicerata
Class:	Arachnida
Order:	Araneae
Infraorder:	Araneomorphae
Family:	Araneidae
Genus:	Mastophora
Species:	M. hutchinsoni
Binomial name
Mastophora hutchinsoni Gertsch, 1955

The Mastophora hutchinsoni, or also known as the American bolas spider is a species of orb weaver and a part of a genus, Mastophora. It is distributed extensively throughout various subtropical geographical areas including Australia, South Africa, Oriental Asia, and the Americas--not found in Europe or South Africa. The phenotypic traits of the female spiders include a large spherical abdomen with a milky white base often covered in darker brown patterns and a smaller brown carapace. In terms of relative size of the species, an extreme sexual size dimorphism allows for a clear distinction between the two sexes, where the female is much larger. Although both sexes hatch with their cephalothorax range being around 0.5 mm, the female develops into a significantly larger specimen than her male counterpart. (insert image)  The size difference coheres with the divergence of their respective hunting behaviors. While female bolas spiders hang from a horizontal thread waiting to swing her bolas or sticky orb towards her nearby prey (typically moths), the males do not use a bolas but rather implement hunting tactics reminiscent of early-instar spiders. Along with a bolas, the female exploits a unique ability that allows her to aggressively mimic the pheromone composition of moths awaiting to mate to attract male prey.

Population Structure, Speciation, and Phylogeny

The M. hutchinsoni are part of the Mastophora species within the larger Araineida or orb-weaver family. They are in the Arthropoda phylum, the Arachnida class, and the Araneae family. Originally stemming from normal orb weavers, the species adopted an alternative web design (the bolas). The Mastophora are widely distributed throughout the world in temperate climates except for in Euro Asia and South Africa. These species can be distinguished by extreme sexual dimorphism where the female grows nearly tenfold the size of the male.

Habitat and Distribution

The M. hutchinsoni primarily reside in the northeastern regions of North America near areas of a more temperate climate. The range of locations can be from Minnesota to New Hampshire. Their primary habitats where they nest or feed include shrubs or short trees which coincide with their hunting behavior and web production as leaves or twigs are necessary for residence outside of hours of predation or for the placement of egg sacs. Due to the limited range of prey affected by the female’s aggressive mimicry technique, the predator frequently moves to different locations according to prey availability. They do so by riding a silken thread that travels with the direction of the wind.

Diet

Adult Female Diet

Lacinipolia renigera

As a result of a highly specialized hunting technique that mimics the disparate chemical compositions of its prey, the hunting range for the M. hutchinsoni is limited to a couple families of male moths. The females typically hunt either Noctuidae moths consisting of the bristly cut-worm, bronzed cutworm, and smoky tetanolita or the bluegrass worm of the Pyralidae family during prime activity hours in the evening--resulting in a nocturnal hunting pattern. In particular, the Lacinipolia renigera (bristly cut-worm) constitute two-thirds of total prey biomass and together with the Tetanolita mynesalis (smoky tetanolita) make up to ninety percent of its overall diet.

Aggressive Pheromone Mimicry

Female M. hutchinsoni employ a specialized hunting tactic where she aggressively mimics the pheromones of moths to attract male moths near their bolas trap. Female moths produce a complex pheromone typically combining two compounds in a ratio specific to the species. This allows male moths to easily distinguish between mates of different species. The periods of activity for each moth are also different. For instance, the bristly cut-worm is active earlier in the night than the smoky tetanolita. Thus, the female spiders must adjust the level of chemicals produced to accommodate for different moths.

Spiderling Diet

Instead of producing bolas like adult females to capture their prey, the spiderlings remain motionless in the margins of leaves in shrubs or trees and wait to quickly ambush small flies that pass. In preparation for capture, the early-instar spiders extend both of their front legs which contain small bristles that aid in capture. These immature spiders also seem to attract male Psychoda using a mechanism near to or the same as aggressive chemical mimicry.

Adult Male Diet

The males also do not construct a bolas to capture. Although females lose the strong bristles on their front legs--used to aid in capture--after maturation, the males retain such features. There are not many observations made on the predatory habits of the male, however there has been research that proved the reservation of juvenile tactics and association with the male and psychodid fly attraction--the main source of the male’s diet.

Webs

Bolas Web Design

Dissimilar to many of the web designs constructed by its closely related species in the family of orb weavers, M. hutchinsoni do not form the typical two-dimensional orb web. Instead, the females primarily reside in a nearby leaf or plant while producing a single horizontal thread which they stay on and travel across during times of predation. At the terminal end, the spider forms an extremely adhesive orb which it quickly hurls in a circular pattern to capture flying prey. The bolas is much more efficient at capturing moths as it prevents the prey from escaping through shedding its scales--an escape mechanism that would otherwise work on typical orb webs. If there is no prey captured after a short while, the Mastophora brings the orb back and consumes it --preparing to form another one to replace the former. Such behavior seems attributed to the short-term efficacy of the orb’s stickiness. Accordingly, after a single use of the bolas, the female must form another if willing to hunt more prey.

Parental Care

Bolas Spider egg sac with eggs inside

The egg sacs of the M. hutchinsoni are spherical in shape with an extended stem typically surrounded by multiple protective layers and an off-white silk covering. The globose vessels have a diameter around 8 mm while the connected stem extends up to thirty-six millimeters. With regards to the placement of the egg sac, the base is attached to either a branch of twig.

Mating

While both sexes of the M. hutchinsoni emerge during the spring, the males normally develop much more quickly than females with around a couple of months in between their respective growth. While the males fully develop during the summer months, the females mature around the fall season. During this gap, the mature males often leave their webs and reside in the webs of developing females awaiting to mate.

Protective Coloration

Considering the extreme sexual dimorphism resulting in a noticeably large size, M. Hutchinsoni adult females fashion a more cryptic coloration with a white palette and dark brown patterns. Meanwhile, the male bolas spiders are much smaller with reddish-brown hue. Moreover, the female’s large globular abdomen allows it to resemble bird droppings sitting on leaves. The bolas spider makes use of this resemblance as a defensive mechanism to avoid encounters with potential predators.

Physiology

The female M. hutchinsoni has the ability to produce and emit a chemical mixture that duplicates the sex pheromones found in the female moths during the mating seasons of moth species. Pheromones blends of each moth species are unique and may interfere with each other resulting in reduced efficacy or complete failure mimicry. To accommodate such discrepancies, the bolas spider continuously produces a substandard blend of both pheromones elements and adjusts the amount of emission as time moves from one species’ zone of activity to another.

Moth Pheromone

The pheromone compositions are unique to their respective species of moth--this differentiation helps male moths recognize the correct mating partner. Accordingly, the pheromones usually contain a blend of multiple chemical components. The bristly cutworm which makes up the majority of the diet of the M. hutchinsoni emits a pheromone with (Z)-9-tetradecenyl acetate (Z9-14: Ac) and 3.8 percent (Z,E)-9,12-tetradecenyl acetate (ZE-9, 12-14: Ac). M. Meanwhile, Tetanolita mynesalis, the other main food source, emits a blend with two parts (3Z, 9Z)-(6S, 7R)-epoxy heneocosadiene and one part (3Z, 6Z, 9Z)-heneicosatrience). Nephelodes minians (the bronzed cutworm) holds a chemical composition consisting of a blend of (Z)-11-hexadecenal and (Z)-11- hexadecenyl acetate. M. hutchinsoni is not only able to synthesize the components of different chemical blends but also mimic the exact ratios.

Wing Vibration

Despite being nocturnal hunters, M. hutchinsoni have poor vision and rather rely on prey wing vibrations to trigger bolas construction.The female has tiny hairs on her legs that are sensitive to such vibrations. Using a cabbage looper moth as an intermediate wing vibration rate for the L.renigera and T. mynelasis, researchers found that the bolas spider constructed a bolas a few minutes after being exposed to wing vibrations.

Bites to Animals

The M. hutchinsoni females produce venom to fatally impair her prey. After the M. hutchinsoni successfully attracts and traps the moth prey on the sticky bolas, the spider quickly reels the moth to herself, paralyzes the moth with a venomous bite, and wraps the prey in an envelope of silk to preserve the meal. The venom of Mastophora is not recorded to be dangerous toward humans or large animals.