Monday, May 23, 2011

Superfamily Pentatomoidea – The stink bugs

The insects known more commonly as the stink bugs, shield bugs or chest bugs are all members of the large superfamily Pentomoidea. There are approximently 7000 known species divided up amongst 14 to 15 families (depending on the system preferred by the consulted taxonomist). All are distinguishable by a similar body plan, the structure of their mouth parts and their varying use of chemical defenses.

Members of the true bugs (order Hemiptera), stink bugs are often mistakenly called beetles (order Coleoptera), despite having being members of a distinct order. While there are a number of anatomical differences between the true bugs and the beetles, one of the most distinctive can be seen in their mouth parts. Members of the true bugs all have modified their mouth parts into a sucking proboscis like structure. Composed of fused mandible and maxillae used for piercing and housed within a labium, most stink bugs use this proboscis to feed on the fluids of plants though a few species are predators of other insects.

Another distinct difference between true bugs and beetles can be seen in their wing structure. The beetle’s elytra (fore-wings) are entirely hardened and are not used for flight. These form a protective barrier for their alae (hind-wings). Both pairs of wings sit side by side and do not overlap, as they are in the true bugs. The true bugs also either have fully membranous wings or partially hardened elytra. In the case of true bugs with partially hardened elytra, the alae will be membranous, but many true bugs with membranous elytra will posses either reduced or completely lack their alae.

The final primary difference can be seen in their life cycle. Beetles are endopteryotes, meaning they go through a complete metamorphosis from a larval stage where as the true bugs are hemimetabolous and, after hatching, their young often appear as under-developed adults, known as nymphs. Often lacking wings until being gained during a molting (the most well known case of this, and also a member order Hemiptera, is the cicada) stink bug nymphs still possess the ability to produce the noxious chemicals found in the adults, though sometimes in a less developed way.

All members of superfamily Pentomoidea (which gets its name from these insects trade-mark 5 segmented antennae) either have a rounded or somewhat triangular body plan. Most in North America are more familiar with the angular body plan, but both forms are common. One of the easiest ways to tell if you are dealing with a member of this superfamily (other than the often nauseating odor when they are disturbed) is their well developed scutellum. This is a hardened extension of the thorax that covers the abdomen and protects part of the insect’s body and its wings, it is also the source of the common names ‘shield bug’ and ‘chest bug’. The scutellum is often mistaken as the hardened fore-wings of the beetles despite not being a part of the wing structures. Instead, the fore-wings are partially hardened and leathery to cover the membranous hind-wings, which are the primary source of flight. This combination of hardened and membranous wings gives the insect its characteristic buzzing sound while in flight.

Because of many stink bugs feeding off of the fluids of plants, many are considered major agricultural pests. A prime example of this can be seen with the introduction of the Brown Marmorated Stink Bug (Halyomorpha halys) to the North-Eastern United States from various parts of East Asia where it is also a common agricultural pest. First spotted in 1988, this species has quickly spread and now threatens crops as diverse as apples, sweet corn, peaches, soy beans, blackberries and others. Besides the damage to crops, many species of stink bugs are also common household pests where their repulsive sent makes many homeowners unsure of how to remove them without causing the insects to release their defensive chemicals. Despite being a common pest, some stink bugs are desirable due to their predatory nature, often feeding off of insects considered pests themselves.

The Brown Marmorated Stink Bug.

Many stink bugs take advantage of a survival technique known as clustering. This occurs when multiple insects congregate to increase their chances of survival as a group and, in some species, to mate. This form of aggregation is often aided by pheromones many species use to attract other individuals and mates. If these pheromones are present, they are typically produced by exocrine glands on the abdomen. It seems a wise maneuver to move in numbers when many animals are afraid to take on one insect, let alone risk a potential sudden cloud of terribly smelling chemicals.

But I’m sure what many of you have been waiting to read involves the source of their most common of names. This has to do with a defensive measure that most members of superfamily Pentatomoidea share to one degree or another, the use of chemical weaponry. Housed in special glands found on either side of the thorax, these insects emit of spray of odiferous chemicals. The nature of these chemicals varies from specie to specie with the most common derivatives being alcohols, aldehydes and esters. In a few species, the compound is based off of cyanide and has the poison’s trademark almond like scent, though despite being based on a potent toxin, exposure is not fatal to humans. It should be noted that there are a few insects that also posses foul smelling chemical defences that have no relation to the true stink bugs. These include the pinacate beetles (genus Eleodes) and the Box Elder Bug (Boisea trivittata) amongst others.

While few can tolerate the rancid smell if allowed to spray, some people have found another use for these pungent insects. As astonishing as it might sound for an insect notorious for its scent, many stink bugs are used in various cuisines from around the globe. Most commonly eaten in Vietnam, Laos and Mexico, they are often prized for their strong flavor.

Thank you Holte Ender for the request!
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