Entomophily orinsect pollination is a form ofpollination wherebypollen of plants, especially but not only offlowering plants, is distributed byinsects. Flowers pollinated by insects typicallyadvertise themselves with bright colours, sometimes with conspicuous patterns (honey guides) leading to rewards of pollen andnectar; they may also have an attractive scent which in some casesmimics insectpheromones. Insect pollinators such asbees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required thecoevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups. Both the size and the density of a population are known to affect pollination and subsequent reproductive performance.[1]
The earlyspermatophytes (seed plants) were largely dependent on thewind to carry their pollen from one plant to another. Prior to the appearance of flowering plants somegymnosperms, such asBennettitales, developed flower-like structures that were likely insect pollinated.[2] The earliest fossil evidence of insect pollination for gymnosperms is from the earlyPermian period (Kungurian stage) based on five compression fossils ofTillyardembia from Russia, which show the gymnosperm pollen attached to their heads, thoraces, legs and abdomens.[3] Candidates for pollinators include extinct longproboscis insect groups, includingAneuretopsychid,Mesopsychid andPseudopolycentropodid scorpionflies,[4]Kalligrammatid[5][6] andParadoxosisyrine[7] lacewings andZhangsolvid flies,[8] as well as some extant families that specialised on gymnosperms before switching to angiosperms, includingNemestrinid,Tabanid andAcrocerid flies.[9] Livingcycads havemutualistic relationships with specific insect species (typically beetles) which pollinate them. Such relationships extend back to at least the late Mesozoic, with bothoedemerid beetles (which today are exclusively found on flowering plants)[2] andboganiid beetles[10] (which still pollinate cycads today) from the Cretaceous being found with preserved cycad pollen.Angiosperms (flowering plants) first appeared during the Early Cretaceous, and during theangiosperm radiation from 125 to 90 Ma, would displace many of the gymnosperm lineages and cause the extinction of many of their pollinators, while some would transition to angiosperms and some new families would form pollination associations with angiosperms.[2] Traits such assapromyophily (emitting the odour ofcarrion to attract flies) have evolved independently in several unrelated angiosperm families.[11]
Wind and water pollination require the production of vast quantities of pollen because of the chancy nature of its deposition. If they are not to be reliant on the wind or water (for aquatic species), plants need pollinators to move their pollen grains from one plant to another. They particularly need pollinators to consistently choose flowers of the same species, so they have evolved different lures to encourage specific pollinators to maintain fidelity to the same species. The attractions offered are mainly nectar, pollen, fragrances and oils. The ideal pollinating insect is hairy (so that pollen adheres to it), and spends time exploring the flower so that it comes into contact with the reproductive structures.[12]
Many insects arepollinators, particularlybees,Lepidoptera (butterflies andmoths),wasps,flies,ants andbeetles.[12] On the other hand, some plants are generalists, being pollinated by insects in several orders.[14] Entomophilous plant species have frequently evolved mechanisms to make themselves more appealing to insects, e.g., brightly coloured or scentedflowers,nectar, or appealing shapes and patterns. Pollen grains of entomophilous plants are generally larger than the fine pollens ofanemophilous (wind-pollinated) plants, which has to be produced in much larger quantities because such a high proportion is wasted. This is energetically costly, but in contrast, entomophilous plants have to bear the energetic costs of producing nectar.[15]
Butterflies and moths have hairy bodies and longproboscides which can probe deep into tubular flowers. Butterflies mostly fly by day and are particularly attracted to pink, mauve and purple flowers. The flowers are often large and scented, and the stamens are so-positioned that pollen is deposited on the insects while they feed on the nectar. Moths are mostly nocturnal and are attracted by night-blooming plants. The flowers of these are often tubular, pale in colour and fragrant only at night.Hawkmoths tend to visit larger flowers and hover as they feed; they transfer pollen by means of the proboscis. Other moths land on the usually smaller flowers, which may be aggregated into flowerheads. Their energetic needs are not so great as those of hawkmoths and they are offered smaller quantities of nectar.[16]
Inflorescences pollinated by beetles tend to be flat with open corollas or small flowers clustered in a head with multiple, projecting anthers that shed pollen readily.[12] The flowers are often green or pale-coloured, and heavily scented, often with fruity or spicy aromas, but sometimes with odours of decaying organic matter. Some, like thegiant water lily, include traps designed to retain the beetles in contact with the reproductive parts for longer periods.[17]
Unspecialised flies with short proboscides are found visiting primitive flowers with readily accessible nectar. More specialised flies likesyrphids andTabanids can visit more advanced blooms, but their purpose is to nourish themselves, and any transfer of pollen from one flower to another happens haphazardly. The small size of many flies is often made up for by their abundance, however they are unreliable pollinators as they may bear incompatible pollen, and lack of suitable breeding habitats may limit their activities. SomePterostylis orchids are pollinated by midges unique to each species. Due to mutual specialisation, pollinators are highly dependent on floral diversity. Therefore, losses in plant diversity, such as those carried on by increasing land use, may be linked to extinctions of pollinators.[18] A decline, for whatever reason, to one side of this partnership can be catastrophic for the other.[19]
Flowers pollinated by bees and wasps vary in shape, colour and size. Yellow or blue plants are often visited, and flowers may have ultra-violet nectar guides, that help the insect to find the nectary. Some flowers, like sage or pea, have lower lips that will only open when sufficiently heavy insects, such as bees, land on them. With the lip depressed, the anthers may bow down to deposit pollen on the insect's back. Other flowers, like tomato, may only liberate their pollen bybuzz pollination, a technique in which abumblebee will cling on to a flower while vibrating its flight muscles, and this dislodges the pollen. Because bees care for their brood, they need to collect more food than just to maintain themselves, and therefore are important pollinators.[19] Other bees are nectar thieves and bite their way through the corolla in order to raid the nectary, in the process bypassing the reproductive structures.[12]
Ants are not well adapted to pollination but they have been shown to perform this function inPolygonum cascadense and in certain desert plants with small blossoms near the ground with little fragrance or visual attraction, small quantities of nectar and limited quantities of sticky pollen.[19]
Some plant speciesco-evolved with a particular pollinator species, such as thebee orchid. The species is almost exclusivelyself-pollinating in its northern ranges, but is pollinated by thesolitary beeEucera in the Mediterranean area. The plant attracts these insects by producing a scent that mimics the scent of the female bee. In addition, the lip acts as a decoy, as the male bee confuses it with a female that is visiting a pink flower. Pollen transfer occurs during the ensuingpseudocopulation.[20]
Figs in the genusFicus have amutualistic arrangement with certain tinyagaonid wasps. In thecommon fig, the inflorescence is asyconium, formed by an enlarged, fleshy, hollowreceptacle with multipleovaries on the inner surface. A female wasp enters through a narrow aperture, fertilizes these pistillate flowers, and lays its eggs in some ovaries, with galls being formed by the developing larvae. In due course, staminate flowers develop inside the syconium. Wingless male wasps hatch and mate with females in the galls before tunnelling their way out of the developing fruit. The winged females, now laden with pollen, follow, flying off to find other receptive syconia at the right stage of development. Most species of fig have their own uniquecommensal species of wasp.[21]
The word is artificially derived from theGreek: εντομο-,entomo-[22] "cut in pieces, segmented", hence "insect"; andφίλη,phile, "loved".
Wind pollination is the reproductive strategy adopted by thegrasses,sedges,rushes andcatkin-bearing plants. Other flowering plants are mostly pollinated by insects (or birds or bats), which seems to be the primitive state, and some plants have secondarily developed wind pollination. Some plants that are wind pollinated have vestigial nectaries, and other plants likecommon heather that are regularly pollinated by insects, produce clouds of pollen and some wind pollination is inevitable. Thehoary plantain is primarily wind pollinated, but is also visited by insects which pollinate it.[15] In general, showy, colourful, fragrant flowers likesunflowers,orchids andBuddleja are insect pollinated. The only entomophilous plants that are notseed plants are the dung-mosses of the familySplachnaceae.[23]
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