Simple eyes in arthropods Simple eye in invertebrates

1 simple eyes in arthropods

1.1 spider eyes
1.2 dorsal ocelli
1.3 stemmata

simple eyes in arthropods
spider eyes

this jumping spider s main ocelli (center pair) acute. outer pair secondary eyes , other pairs of secondary eyes on sides , top of head.



head of wasp 3 ocelli (centre), , dorsal part of compound eyes (left , right)

spiders not have compound eyes, instead have several pairs of simple eyes each pair adapted specific task or tasks. principal , secondary eyes in spiders arranged in 4 or more pairs. principal eyes have moveable retinas. secondary eyes have reflector @ of eyes. light-sensitive part of receptor cells next this, direct , reflected light. in hunting or jumping spiders, example, forward-facing pair possesses best resolution (and telescopic components) see (often small) prey @ large distance. night-hunting spiders eyes sensitive in low light levels large aperture, f/0.58.

dorsal ocelli

the term ocellus (plural ocelli) derived latin oculus (eye), , literally means little eye . 2 distinct ocellus types exist: dorsal ocelli (or ocelli ), found in insects, , lateral ocelli (or stemmata), found in larvae of insect orders. structurally , functionally different. simple eyes of other animals, e.g. cnidarians, may referred ocelli, again structure , anatomy of these eyes quite distinct of dorsal ocelli of insects.

dorsal ocelli light-sensitive organs found on dorsal (top-most) surface or frontal surface of head of many insects, e.g. hymenoptera (bees, ants, wasps, sawflies), diptera (flies), odonata (dragonflies, damselflies) , orthoptera (grasshoppers, locusts). ocelli coexist compound eyes; thus, insects possess 2 anatomically separate , functionally different visual pathways.

the number, forms, , functions of dorsal ocelli vary markedly throughout insect orders. tend larger , more expressed in flying insects (particularly bees, wasps, dragonflies , locusts), typically found triplet. 2 lateral ocelli directed left , right of head, respectively, while central (median) ocellus directed frontally. in terrestrial insects (e.g. ants , cockroaches), 2 lateral ocelli present: median ocellus absent. unfortunately labelled lateral ocelli here refers sideways-facing position of ocelli, of dorsal type. should not confused lateral ocelli of insect larvae (see stemmata).

a dorsal ocellus consists of lens element (cornea) , layer of photoreceptors (rod cells). ocellar lens may curved (e.g. bees, locusts, dragonflies) or flat (e.g. cockroaches). photoreceptor layer may (e.g. locusts) or may not (e.g. blowflies, dragonflies) separated lens clear zone (vitreous humour). number of photoreceptors varies widely, may number in hundreds or thousands well-developed ocelli.

two unusual features of ocelli particularly notable , conserved between insect orders.

these 2 factors have led conclusion dorsal ocelli incapable of perceiving form, , solely suitable light-metering functions. given large aperture , low f-number of lens, high convergence ratios , synaptic gains, ocelli considered far more sensitive light compound eyes. additionally, given relatively simple neural arrangement of eye (small number of synapses between detector , effector), extremely large diameter of ocellar interneurons (often largest diameter neurons in animal s nervous system), ocelli typically considered faster compound eyes.

one common theory of ocellar function in flying insects holds used assist in maintaining flight stability. given underfocused nature, wide fields of view, , high light-collecting ability, ocelli superbly adapted measuring changes in perceived brightness of external world insect rolls or pitches around body axis during flight. corrective flight responses light have been demonstrated in locusts , dragonflies in tethered flight. other theories of ocellar function have ranged roles light adaptors or global excitatory organs polarization sensors , circadian entrainers.

recent studies have shown ocelli of insects (most notably dragonfly, wasps) capable of form vision, ocellar lens forms image within, or close to, photoreceptor layer. in dragonflies has been demonstrated receptive fields of both photoreceptors , second-order neurons can quite restricted. further research has demonstrated these eyes not resolve spatial details of world, perceive motion. second-order neurons in dragonfly median ocellus respond more upwards-moving bars , gratings downwards-moving bars , gratings, effect present when ultraviolet light used in stimulus; when ultraviolet light absent, no directional response observed. dragonfly ocelli highly developed , specialised visual organs, may support exceptional acrobatic abilities of these animals.

research on ocelli of high interest designers of small unmanned aerial vehicles. designers of these craft face many of same challenges insects face in maintaining stability in three-dimensional world. engineers increasingly taking inspiration insects overcome these challenges.

stemmata

moth larva moult; new stemmata visible behind old head capsule



an example of sawfly larva. has single pair of stemmata, , set higher on head position of stemmata on heads of lepidopteran larvae



the larva of 1 of acherontia species shown here, typical of order lepidoptera. head of larva bears more 1 pair of stemmata, of set low down , far more placed mouthparts

stemmata (singular stemma) class of simple eyes. many kinds of holometabolous larvae bear no other form of eyes until enter final stage of growth. adults of several orders of insects have stemmata , never develop compound eyes @ all. examples include fleas, springtails, , thysanura. strepsiptera have clusters of simple eyes). other arthropoda, such myriapoda, never have eyes other stemmata @ stage of lives.

behind each lens of typical, functional stemma, lies single cluster of photoreceptor cells, termed retinula. lens biconvex, , body of stemma has vitreous or crystalline core.

although stemmata simple eyes, kinds, such of larvae of lepidoptera , of tenthredinidae, family of sawflies, simple in represent immature or embryonic forms of compound eyes of adult. can possess considerable degree of acuity , sensitivity, , can detect polarized light. in pupal stage, such stemmata develop fledged compound eyes. 1 feature offering clue ontogenetic role lateral position on head; ocelli, in other ways resemble stemmata, tend borne in sites median compound eyes, or so. in circles distinction has led use of term lateral ocelli stemmata.

a scolopendra species (chilopoda) stemmata incompletely aggregated compound eyes