Red Flour Beetle
The insecticidal toxins produced by the bacterium Bacillus thuringiensis are called Cry (for crystal) toxins. The toxins are numbered in a way that uniquely identifies each toxin. Cry3Aa is a toxin that affects beetles and not other species. Why is it so specific?
The answer requires knowledge of the Cry protein. Cry proteins can insert into cell membranes to form pores, channels that allow ions and water to cross the membrane. Mass ion movement can upset the osmotic balance of the cell. In a susceptible insect, the Cry toxin forms pores in the cells of the gut, disrupting the osmotic balance resulting in the swelling and bursting of cells. The cell destruction causes a breach in the insect gut that allow gut bacteria to infect the hemolymph of the insect. The infection can kill the insect.
A Cry toxin alone will not readily insert into membranes. It needs help from the target cell. Cry toxins have 3 domains. Domain I forms the membrane pores. Domains II and III contain a series of loops and other structures that can bind to proteins on the surface of the target cell. It is the binding properties of Domains II and III that determine BT specificity.
Cry3Aa specifically affects some beetles and is toxic to the red flour beetle, Tenebrio molitor. Cells in the gut of the red flour beetle contain proteins called “cadherins”, short for calcium containing proteins responsible for cell adhesion (sticking together). The structure and binding properties of cadherins vary among species. The cadherin of the red flour beetle binds the Domain II of the Cry3Aa toxin in a way that facilitates insertion of the pore forming region into the cell membrane. Insects and species that have cadherins with a different structure do not bind the Cry3Aa and it does not readily insert into the membranes of cells in those species. Thus Cry3Aa can easily insert into the membranes and is quite toxic to red flour beetle. The Cry3Aa does not significantly insert into membranes of non-susceptible species and it is therefore not toxic to bees, butterflies, cows and people.
American Burying Beetle Photo: Doug Blacklund
Parasites are often adapted for clinging to a host. These features sometimes reduce mobility because features that might improve mobility could make the parasite easier to dislodge. Parasites need to move from one host to another when dispersing after reproduction or in instances when a host dies. Parasites with limited mobility need creative ways to locate hosts. Ixodes ricinus,
a common tick species in Europe can feed on a variety of host vertebrates. When an animal dies, necrophilous beetles will arrive at a carcass to feed or lay eggs. A study of tick movement in Spain* discovered that I. ricinus
can hitch rides on beetles. The ticks were found attached to the elytra of the heath dumble dor beetle Trypocopris pyrenaeus
, the burying beetle, Nicrophorus vespilloides
and the dung beetle, Anoplotrupes stercorosus.
These beetles all fly to the animals themselves or the areas the animals inhabit. For the tick, hitching a ride beats walking.
*Saloña-Bordas, Marta; Bahillo de la Puebla, Pablo; Díaz Martín, Beatriz; Sumner, Jason & Perotti, M.Alejandra. 2015. Ixodes ricinus (Ixodidae), an occasional phoront on necrophagous and coprophagous beetles in Europe. Experimental and Applied Acarology. 65:243-248.
Some Species Are Adapted to Winter Flooding
Winter flooding is common in many of the temperate areas of North America and Europe. What happens to overwintering insects when it floods? Adis and Junk* reviewed the ability of beetles to survive submersion. Staphylinids, common in areas with winter flooding can withstand 30 days under water at low temperature. The carabid, Bembidion dentellum
, can survive submerged under water for over 40 days at low temperatures (4 C). At 17 C, the beetles will emerge after only 2 days. Why does temperature have an effect? At low temperature the insects remain in diapause, a state of low respiration (oxygen requirement). At higher temperatures, the insects may break diapause, increase respiration and be unable to withstand extended submersion. It is possible that many insects overwintering in flooded soils may diapause with a very low respiration rate. This allows these insects to survive anaerobic conditions that may persist for weeks in flood prone areas. Flooding may exclude other species that are not adapted. Thus, winter flooding may be a factor that impacts species diversity and distribution. Global Climate Change may lead to more and more prolonged flooding in some areas. This is one way that climate change may affect species composition.
*JOACHIM ADIS and WOLFGANG J. JUNK. 2002. Terrestrial invertebrates inhabiting lowland river floodplains of Central Amazonia and Central Europe: a review. Freshwater Biology 47:711–731.
American Burying Beetle
Photo: Doug Blacklund
The American Burying Beetle, Nicrophorus americanus,
is an endangered species with limited habitat remaining. Eastern Oklahoma contains significant habitat that must be preserved or mitigated. Mitigation typically takes the form of a company or organization establishing a permanent habitat area approved by the US-FWS. The protected habitat can generate income for American Burying Beetle conservation by selling mitigation credits to other parties who wish to develop land in a way that negatively impacts the beetle. A typical credit sells for around $10,000 per acre. The mitigation requirement can significantly increase the initial costs of development. Some governmental units in Oklahoma, object
to the increased cost and would like the Burying Beetle removed from the endangered species list. Perhaps they should instead re-evaluate their development goals? Can they develop by infill of existing already developed areas instead? Do they really need to widen the current roads and bridges?
The cost of beetle protection is a true environmental cost that must be paid by someone. In the past, people and projects could destroy beetle habitat with no monetary cost to themselves, by transferring the costs to others or degrading the ecosystem. Now the cost has become part of the cost of doing business. Special interests can fight the process or they can comply and reevaluate their own processes to minimize the impact. Sprawl and development have real costs for the environment. Sometimes it takes an endangered species to bring that cost into our collective conscience.
Illustration: H.G. Hubbard
The gopher tortoise, Gopherus polyphemus
is a long lived tortoise that digs burrows deep underground. The tortoise is considered threatened and is protected under US-FWS
rules. The burrows are sizable and invite over 350 species of animal “guests” including many species of insects. In 1894 Henry G Hubbard published The Insect Guests of the Florida Land Tortoise
. Hubbard was interested in a rare frog that was reported to inhabit the tortoise burrow. He began excavation of a tortoise burrow in 1893, an undertaking that required much digging and the creation of a large pit. Hubbard found his frog and numerous other inhabitants that he also collected. His findings encouraged him to excavate more burrows: eight in total.
8 Species of Beetles, including a coprophagous species feeding on turtle droppings.
1 Undescribed species of caterpillar that feeds on turtle dung. Attempts at rearing the species to adulthood were unsuccessful.
1 Species of Wingless Cricket.
2 Species of pseudoscorpion.
2 Species of ticks.
Many of the species encountered lacked eyes and possessed long sensory bristles- adaptations for the dark environment. Many of the new species described by Hubbard had close relatives that were adapted to life above ground indicating a recent association with the burrows of tortoises,
The “Hanging Thief”, Robber fly
The fly haltere is a remarkable structure that changes the function of the hind wing from motion production into a sensor. Most insects move their wings by vibrating the thorax. Both forewing and hindwing move up and down in phase. A fly that beats its wings several hundred times per second will move its halteres at the same rate. Instead of a wing shape similar to the forewing, a fly haltere is a stalk with a ball on the end. When a fly turns in flight, the turn places a force on the haltere due to the Coriolis effect. (The haltere tends to keep moving in the same plane and the turn causes the haltere to bend relative to the fly’s body.) The force on the haltere stretches the sensor at its base during a turn. This signals to the fly that its direction of motion has changed.
A flying object can rotate around one of three axes defined as roll, pitch and yaw. Although the force on a haltere can be applied along all 3 axes, the measurement of that force is made by the two-dimensional stretch receptor at the base of the haltere. A single haltere has only two measuring axes. However, halteres are not positioned in a linear fashion and do not beat in parallel planes. They are typically offset by 120 degrees instead of 180 degrees. Thus, the halteres sense movements along two horizontal axes and two vertical axes. The separate signals from the two halteres are integrated in the fly nervous system to give roll, pitch and yaw information. The integration makes a fly more sensitive to changes in pitch than changes in roll.
A fly with one haltere can get some information on change in its flight attitude so it can still fly. A fly with no halteres is flying without sensors. The fly uses feedback from the halteres to make adjustments in body position that keep it airborne. Without information, the fly cannot prevent assuming unstable postures that cause it to fall through from the air.
The idea of halteres acting like the balancers used by a ropewalker
came under criticism over time. The insect physiologist, Gottfried Frankel, summarized the popular theories and critiqued them in his 1939 paper, The Function of the Halteres of Flies.* Frankel listed the following facts that any theory must address:
(1) After removal of the halteres equilibrium during flight is upset, and as a consequence of this the fly cannot keep in the air.
(2) The flies are perfectly able to take to flight and to fly for long periods when suspended in mid-air.
(3) Removal of one haltere alone has either no effect at all on flight or only a very slight one.
(4) Immobilization of the halteres by sticking them fast with a small drop of glue has the same effect as complete removal. This has been previously reported by Weinland (1891), and confirmed by other authors.
(5) Cutting off the end knob alone has the same effect as removal of the entire haltere including the base.
(6) The halteres are vibrating rapidly during flight in a plane which is fixed relative to the body of the fly.
These facts argued against the balancer theory. If a fly with only one haltere can fly relatively normally, this discounts the idea of halteres acting as balancers. Earlier critics had noted that the weight of the halteres is not significantly large to act as a balancer.
Frankel proposed that the halteres must work as a sense organ to stabilize flight. They vibrate up and down with the fly wingbeat in a single plane. Changes in attidude of the fly during flight would apply force to the halteres which would send feedback to the fly. Our modern understanding of haltere function is built on this simple idea.
*Fraenkel, G. (1939). The function of the haltere of flies (Diptera). Proc. Zool. Soc. Lond. A109, 69-78.