Insect pollinators, attracting

Many flowering plants attract insect pollinators by releasing odorant molecules that mimic an insect s natural food sources or potential egg-laying sites. Plants pollinated by flies or beetles that normally feed on or lay their eggs in dung or carrion sometimes use foulsmelling compounds to attract these insects. 

Not all floral scents consist of fragrant volatiles. Unpleasant odors associated with dung, carrion, urine, rotting fungi, and decaying cabbage and onion have been found in abiotic mimicry in many plant families such as Araceae, Aristolochiaceae, Orchidaceae, and Apocynaceae.103 Several saprophagous insects are attracted to these flowers and act as selective pollinators. 

Insect-pollinated plants are called Entomopkilous. These, being dependent upon the visits of insects for fertilization, possess brilliantly colored corollas, have fragrant odors, and secrete nectar, a sweet liquid very attractive to insects, which are adapted to this work through the possession of a pollen-carrying apparatus. Example Orchids. 

This is accomplished using the alternative oxidase system (Box D in Fig. 18-6). The lotus flower maintains a temperature of 30-35°C, while the ambient temperature may vary from 10-30°C. While the volatilization of insect attractants may be the primary role for thermogenesis in plants, the warm flowers may also offer an important reward to insect pollinators. Beetles and bees require thoracic temperatures above 30°C to initiate flight and, therefore. 

A wild tobacco plant from the Mojave Desert in the United States Nicotiana attenuata) uses very low doses of nicotine in its nectar. The plant uses sweet-smelling benzylacetone to attract insect pollinators, but the bitter taste of the very small amounts of nicotine deters visitors such as hawkmoths and hummingbirds from taking too much nectar. 

The function of the essential oil in the plant is not fully understood. Microscopic examination of plant parts that contain the oil sacs readily shows their presence. The odors of flowers are said to act as attractants for insects involved in pollination and thus may aid in preservation and natural selection. Essential oils are almost always bacteriostats and often bacteriocides. Many components of essential oils are chemically active and thus could participate readily in metaboHc reactions. They are sources of plant metaboHc energy, although some chemists have referred to them as waste products of plant metaboHsm. Exudates, which contain essential oils, eg, balsams and resins, act as protective seals against disease or parasites, prevent loss of sap, and are formed readily when the tree tmnks are damaged. 

Apocarotenoids also act as chemoattractants, repellants, and growth effectors in plants and cyanobacteria. They attract pollinators to plants through the use of color similar to full-length carotenoids. Their aromas are thought to be attractants for animals and insects to facilitate in seed dispersal and pollination. Small volatile apocarotenoids lure pollinators and levels of apocarotenoids... 

The predators discussed up to this point search for prey by using their ability to perceive certain chemical clues. Some unusual predators have evolved the ability to attract their prey with scents that mimic the odor of a valuable resource (see reviews of chemical mimicry in refs. 9 and 39). Several groups of spiders lure male insect prey with scents that mimic the sex pheromone scents of females of the prey species (see reviews in refs. 9,13,40, and 41). To the best of our knowledge, these spiders are the only predators that mimic sex pheromones. However, the spiders share some similarities with the diverse orchids which mimic insect sex pheromones to lure pollinators (9, 42, 43) and with the predatory fireflies, which practice elaborate mimicry of visual sexual signals to lure their prey heterospecific male fireflies (44). 

Both moth and plant are well adapted to their lifestyles. Because most yuccas produce little nectar, they offer no reward to attract other insects but entrust their pollination solely to yucca moths. The lack of nectar in yucca flowers means nothing to the moths. They have an incomplete gut and cannot feed in any event. Without nourishment, they live only long enough to fulfill their reproductive tasks. Two to four days after emerging from their cocoons, they die. 

Secondary compounds produced by plants may have other significant survival roles, such as signals to attract insects, birds, or other animals to enhance pollination or seed dispersal. In addition to any potential functions, secondary compounds may concomitantly serve a physiologic function, such as protection against ultraviolet (UV) light or frost, or provide a function in nitrogen transport and storage. In several instances, compounds can serve multiple functions in the same plant. Anthocyanins or monoterpenes can be... 

Numerous nitrogen-free toxins occur in plants. As discussed in the introduction to this Chapter, many of these compounds are believed to be for the protection of the plant from herbivory. However, because there is such a diversity in plant compounds, there are other functions they serve, e.g., insect attractants for pollination, and protection against environmental factors, such as UV light, low or high temperatures, drought, etc. 

---