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Form of pollination by insects

Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns (honey guides) leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees 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 the coevolution 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.

The early spermatophytes (seed plants) were largely dependent on the wind to carry their pollen from one plant to another. Prior to the appearance of flowering plants some gymnosperms, such as Bennettitales, developed flower like structures that were likely insect pollinated. Insects pollination for gymnosperms likely originated in the Permian period. Candidates for pollinators include extinct long proboscis insect groups, including Aneuretopsychid, Mesopsychid and Pseudopolycentropodid scorpionflies, Kalligrammatid and Paradoxosisyrine lacewings and Zhangsolvid flies, as well as some extant families that specialised on gymnosperms before switching to angiosperms, including Nemestrinid, Tabanid and Acrocerid flies. Living cycads have mutualistic relationships with specific insect species (typically beetles) which pollinate them. Such relationships extend back to at least the late Mesozoic, with both oedemerid beetles (which today are exclusively found on flowering plants) (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 the angiosperm 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.

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. On the other hand, some plants are generalists, being pollinated by insects in several orders. Entomophilous plant species have frequently evolved mechanisms to make themselves more appealing to insects, e. g., brightly coloured or scented flowers, nectar, or appealing shapes and patterns. Pollen grains of entomophilous plants are generally larger than the fine pollens of anemophilous (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. 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. Unspecialised flies with short proboscides are found visiting primitive flowers with readily accessible nectar. More specialised flies like syrphids and Tabanids 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. Some Pterostylis 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. A decline, for whatever reason, to one side of this partnership can be catastrophic for the other. 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. Figs in the genus Ficus have a mutualistic arrangement with certain tiny agaonid wasps. In the common fig, the inflorescence is a syconium, formed by an enlarged, fleshy, hollow receptacle with multiple ovaries 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 unique commensal species of wasp.

The word is artificially derived from the Greek: εντομο , entomo "cut in pieces, segmented", hence "insect"; and φίλη, phile, "loved".

Wind pollination is the reproductive strategy adopted by the grasses, sedges, rushes and catkin 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 like common heather that are regularly pollinated by insects, produce clouds of pollen and some wind pollination is inevitable. The hoary plantain is primarily wind pollinated, but is also visited by insects which pollinate it. In general, showy, colourful, fragrant flowers like sunflowers, orchids and Buddleja are insect pollinated. The only entomophilous plants that are not seed plants are the dung mosses of the family Splachnaceae.