Blaberus giganteus

Giant cockroach
Blaberus giganteus - Female
Blaberus giganteus - Female
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Blattodea
Family:	Blaberidae
Genus:	Blaberus
Species:	B. giganteus
Binomial name
Blaberus giganteus (Linnaeus, 1758)

The Central American Giant Cockroach, Blaberus giganteus, is considered one of the largest cockroaches in the world, with males being able to reach lengths of 7.5 cm and females 10 cm.^[1] This cockroach belongs to the family Blaberidae. As typical for all roaches, individuals undergo hemimetabolous metamorphosis, which means the change from juvenile to adult is gradual.^[2] It is endemic to Central America and northern South America, and can be found naturally occurring in the rainforests.^[3] Habitat preferences include areas of high moisture and little light, such as caves, tree hollows, cracks in rocks, etc.^[3] Their lifespan can be up to 20 months depending on habitat conditions and diet.^[4] The majority of its diet is decaying plant material, but Blaberus giganteus is an omnivore and a scavenger.^[1] Other food choices can include bat guano, fruit, seeds, dead insects, and other dead animals.^[1]

Mating

Two chemical signals play important roles in the sexual behaviour of B. giganteus.^[5] The sex pheromone is released by the female and used in attracting mates that are long distances away.^[5] The male will produce an aphrodisiac sex hormone from its tergal glands that will encourage female mounting.^[5] Females choose the males with which they will mate, so this sexual selection becomes a major pressure and driving force behind evolution.^[6] Carbohydrate intake has been found to be related to male sex pheromone expression, dominance status, and attractiveness more so than protein.^[6] Males have been shown to have a preference for a high carbohydrate diet versus one focused on protein.^[6] This would suggest they are actively increasing their carbohydrate consumption to maximize their reproductive fitness and attractiveness to potential female mates.^[6] After mating, the female B. giganteus will be pregnant for life and stores the fertilized eggs in her ootheca, where they are incubated for roughly 60 days.^[5] When the eggs are about to hatch, the female will expel the ootheca so the nymphs can break free and feed on their first meal, which consists of the ootheca.^[5] After eating their fill, the young nymphs will burrow into soil or somewhere dark and remain there until they have molted numerous times and reached maturity.^[5]

Defense against fungal infection

When exposed to infection or invasion of various microorganisms, insects have two general responses of their immune systems. In B. giganteus, such an invasion will elicit a humoral response, where specific proteins are produced or activated by the existence of a pathogen.^[4] The fat body, which is usually associated with storing and releasing energy depending on demands, will induce several novel proteins when confronted with fungal cell walls.^[4] Of interest is that the giant cockroach exhibits adaptive humoral responses.^[4] That means their immune response has a specific memory similar to what can be found in mammalian immune systems.^[4] This is beneficial for long-lived individuals, as they have increased chances of encountering the same infection numerous times.^[4] The biological significance of these proteins has yet to be determined, but they are known to play a role in defense against fungal infections.^[4]

Endosymbiosis

Blaberus giganteus has a special relationship with a genus of obligate flavobacterial endosymbiont called Blattabacterium sp.^[1] They engage in a host microbe relationship.^[1] The microbe's job is to take nitrogenous waste such as urea and ammonia and process it into amino acids that can be used by the cockroach.^[1] This is very beneficial to the giant cockroach because overall its diet is plant based and considered very nitrogen poor.^[1] Even though carbohydrate consumption is beneficial in mating it does not play an active role in male to male competition.^[6]

Growth

Cockroaches grow gradually through hemimetabolous metamorphosis. There are three distinct stages in their life cycle that include egg, nymph, and adult. Only adults are able to reproduce and will have wings.^[6] Prolonged nymphal stages along with additional molts can sometimes occur in B. giganteus for a number of reasons. One hypothesis is that the absence jostling and mutual stimulation which are found often in colony life could slow the developmental process.^[7] In other instances lower temperatures and reduced humidity can lead to delayed maturation and an increase in the number of molts.^[7] This is a response by the insect to unfavourable habitat conditions and can also be seen as a predatory response.

Locomotion

Cockroaches always have three legs in synchronous contact with the ground during movement.^[8] The three legs are classified as the leading leg, middle leg, and trailing leg and the leading and trailing leg from one side with the middle leg of the other side will form a tripod.^[8] The leading leg will pull the body, while the trailing leg pushes the middle leg forward.^[8] The middle leg is important because it will act as a pivot and creates the characteristic zigzag locomotion.^[8] The process will be repeated with the next tripod and to move forward the tripods alternate.^[8] The ability of cockroaches to have ground reaction force distributed equally to these three legs is explained by joint torque minimization.^[8] Joint torque minimization has been shown to help limit mechanical, energy, and metabolism demands, and can also decrease the axial load on a single leg.^[8] Cockroaches can easily walk up a 45˚ slope on a smooth surface with little to no difficulty.^[8] However, aged cockroaches or cockroaches with damaged tarsi can overcome such slopes only with difficulty.

Muscle metabolism and respiratory system

The rate of oxygen consumption in some animals and in insects is proportionate to body weight.^[9] Oxygen consumption will increase with activity and will be subject to rhythmical cycles of activity exhibited in cockroaches.^[9] Because cockroaches do not have lungs to breathe, they take in air through small holes on the sides of the body known as spiracles.^[9] Attached to these spiracles are tubes called tracheae that will branch throughout the body of the cockroach until they associate with each cell.^[9] Oxygen will diffuse across the thin cuticle and carbon dioxide will diffuse out, and this allows cockroaches to deliver oxygen to cells directly without relying on blood like humans.^[9] There can be differences in oxygen consumption between sexes of the same organism. Oxygen consumption in the mixed red and white muscles of mature male B. giganteus was higher when compared to mature females.^[9] This is likely due to sex-related differences of sex hormones causing increased accumulation of oxidized substrates or increased concentration of enzymes in muscles in males.^[9] Males have been shown to have higher levels of glycogen and mitochondria in muscle cells.^[9] Because B. giganteus is so large, they are assumed to have a higher metabolic rate versus other cockroaches, such as Periplaneta americana, but in comparison they are quite sluggish.^[9] Rates of oxygen consumption are significantly higher in P. americana when compared to B. giganteus likely due to higher daily rhythmic activity.^[9]

Hemolymph

Hemolymph is the fluid used in some arthropod circulatory systems, including insects, to fills the interior hemocoel.^[10] Hemolymph is composed of water, inorganic salts, and organic compounds.^[10] Some of the organic compounds are free amino acids, and the content will vary by species in terms of which amino acids are present and their overall concentrations.^[10] The amino acids present in B. giganteus are alanine, arginine, cysteine, glutamic acid, glycine, histidine, leucine, proline, threonine, tyrosine, and valine.^[10] The amino acids present in greatest proportions were glutamic acid, alanine, glycine, and histidine.^[10] The overall concentration of amino acids is roughly 265 mg/100 ml of hemolymph.^[10] The presence of alanine, cysteine, glutamic acid, leucine, proline, tyrosine and valine is shared among different species of cockroaches, such as Blattella germanica and Periplaneta americana.^[10] The presence of arginine, however, is species-specific to B. giganteus.^[10]

References

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