Homeostatic functioning of hymenopterans
In evolution, organisms need to adapt to their surrounding environment in order to survive and multiply. Several factors are considered to influence the success of each species in the wild, these include body size, behavior, phylogeny and environment. The cost of survival for each species requires that the organism’s needs for food, shelter and procreation are well maintained. In this paper, different mechanisms for regulation of physiology functions, better known as homeostasis, in hymenopterans (Kingdom Animalia, Phylum Athropoda, Class Insecta, Order Hymenoptera) will be described is associated with the structure, function and evolution of these organisms.
Respiration is one of the most important processes of living organisms. Each type of organism has a critical requirement for molecular oxygen that can only be acquired from the external environment. Oxygen is then utilized for producing energy in tissue cells that are in turn, working simultaneously with physiological, biochemical and behavioral processes in the body of the organism. The need for this gas is so important that organisms will not survive without a continuous supply of oxygen, Acquisition of oxygen should be design in a very efficient manner to cater to the demands of the body in order for an organism to normally function.
In hymenopterans, the apparatus employed for the acquisition of gas from the environment is so simple yet very efficient. Air sacs assist the direct delivery of oxygen to the tissue cells to the trachea and tracheoles with a negligible decrease in the partial pressure of oxygen. Both respiratory and circulatory systems are totally disconnected, removing the circulatory system from the responsibility for gas exchange. In turn, the spiracles of the tracheal system compress the air and remove exhaust at the same time. The architecture of the air sacs and the trachea are supported by circular cuticular taenidia. Physiologically, the hymenopteran tracheal system is similar to plant leaves, wherein gases are directly transmitted to the cells without the need for a circulatory system. In plants, this gas transmission is performed by stomata, while in hymenopterans, the same function is provided by spiracles.
The tracheal system of hymenopterans is designed with precision that every cell in the body is supplied with oxygen. The tracheoles are very thin branches of the tracheal system, and from an evolutionary point of view, are equivalent to blood capillaries in the vertebrates. In actuality, the tracheoles are capable of providing 10-fold more oxygen per gram of tissue than capillaries. Tissues that are actively involved in metabolic pathways are supplied with very narrow tracheoles that may cause minutes infoldings in some cells. Mitochondria are generally situated around the tracheoles to provide energy in very active regions of the hymenopteran. The tidal volume of the trachea is increased by 70% by the presence of tracheoles, which in turn reduce the need for gas diffusion to the rest of the part of the body (Maina, 2002). The flight muscles of hymenopterans have the highest rates for metabolism, hence the need for a continuous supply of oxygen as an energy source is required.
One of the most familiar gas exchange patterns found in hymenopterans involves the discontinuous gas exchange cycle (DGEC) that involves a repetitive cycle of opening and closing of the spiracles resulting in timed release of carbon dioxide to the environment. It has been observed that these rhythmic releases of carbon dioxide does not ever cease and does not follow any pattern. However, during physical activities such as flights from one spot to another, the spiracle would open and close at a much faster rate as well as remain open for a longer duration to support aerobic metabolism. In addition, it has been determined that temperature has a major effect on rates of gas exchange, wherein environments of higher temperature result in a faster achievement of the critical concentration of carbon dioxide in a hymenopteran organism, mainly due to an increase in its metabolic rate. As this happens, more cyclic or more continuous patterns of gas exchange replace the discontinuous gas exchange cycle (Gray and Bradley, 2006).
Water loss is a perennial problem of hymenopterans, mostly due to their small size and their frequent association with hot and dry conditions such as deserts. The effects of water stress may be serious if it results in a major depletion of the body’s water supply, which is essential for normal physiological function. Fortunately, hymenopterans have evolved ways to go around such environmental problems. Adaptations to such dessicating conditions include the innate decrease in the rate of water loss, the adaptation of the ability to store more water in the body, and thirdly, the acquired ability to tolerate loss of large amounts of water from their bodies. Some specific inserts that are normally located in the deserts are known to be capable of losing as much as 50% of their total body water content. Such extreme dehydration stress can be observed in the aquatic beetle, Peltodytes aquaticus, which has evolved to have a large storage capability for water. Other hymenopterans such as the fruitfly, Drosophila, can be found in a wide range of environments, that it would be interesting to know what mechanisms this species employs to control its rates for water loss. A decrease in the rate of metabolism of an organism has been suggested to be a mode in conserving water by reducing the amount of gas exchange. However, such approach would also require regulation of frequency of spiracle opening, wherein frequent opening of spiracles would result in more water loss.
Research on modes of reduction of water loss based on three routes: firstly, thru excretion from mouthparts and anus, cuticle transpiration, and thirdly, evaporation through open spiracles resulting in water loss (Gibbs et al. 2003). It has been estimated that respiratory water loss via spiracles accounts only for approximately 10% of total body water loss, suggesting the majority of the water lost from the body is thru cuticular evaporation.
In order to deal with water loss, several adaptations have emerged in hymenopterans. These organisms have adapted a tolerance to changes in osmosis gradients and osmotic regulation by sequestration and excretion. Studies on the patterns of osmoregulation in Drosophila are of prime interest because this species has served as a non-vertebrate model organism in several biomedical fields. It has been observed that water loss or dessication frequently occurs in Drosophila because these fly spend most of their time flying around to find mates, food sources and sites to lay eggs. Most of the water content is the fruitfly is stored in the hemolymph, and that the volume decreases during dessication. However, some essential ions, including sodium, chloride, and potassium, are removed from the hemolymph and excreted, possibly to regulate the osmolality or the number of solutes per kg of water) in order to regulate their internal fluids. It seems that the hemolymph serves as a reservoir where water can be stored as well as redistributed to other cells. Such mechanism is essential to the fruitfly, an excess concentration of ions that may result in cellular and metabolic malfunctions as well as cell death. It is interesting to know that the after losing more than two-thirds of its body water, Drosophila has the ability to recover from dessication by rehydrating itself using rainwater or dew, nectar or other fruit juices found around their areas of flight (Albers and Bradley, 2004). Such activity replenishes the hemolymph levels in the body. In addition to an increase in tolerance to dessication, hymenopterans have also evolutionarily adapted strategies in tolerating higher levels of osmolality in their body, probably through the use of heat shock proteins in their bodies. Also, the hymenopterans have also adapted to storage and usage of carbohydrates in their bodies so that during dessication, these inactive sugar sources may be used as source of energy for metabolism as well as solute for maintaining osmolality in their bodies. It has been estimated that the amount of dessication that an organism could withstand is significantly correlated with the body’s carbohydrate content.
More detailed studies are needed to provide a better understanding of the mechanisms for gas exchange and water retention in hymenopterans. Two major osmotic strategies, osmotic regulation and volume homeostasis, are essential in providing more information on the physiological, genetic and evolutionary routes of survival of these organisms.
Albers, M.A. and Bradley, T.J. (2004): Osmotic regulation in adult Drosophila melanogaster during dehydration and rehydration. J. Exp. Biol. 207:2313-2321.
Gibbs, A.G., Fukuzato, F. and Matzkin, L.M. (2003): Evolution of water conservation mechanisms in Drosophila. J. Exp. Biol. 206:1183-1192.
Gray, E.M. and Bradley, T.J. (2006): Evidence from mosquitoes suggests that cyclic gas exchange and discontinuous gas exchange are two manifestations of a single respiratory system. J. Exp. Biol. 209:1603-1611.
Maina, J.N. (2002): Structure, function and evolution of the gas exchangers: Comparative perspectives. J. Anat. 201:281-304.