![]() Issues in Aphid Biology | ![]() | "It has long been an axiom of mine that the little things are infinitely the most important" |
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Aphid Nymphs, Instars, Moults & Metamorphosis:Biology and MorphologyBrightwell, R. & Dransfield, R.D.On this page: Nymphs, instars & moults. Why aphids moult Complete and incomplete metamorphosisAphid nymphs & moultsNymphs are the young of an insect that undergoes incomplete metamorphosis, or no metamorphosis. Most aphids nymphs are borne live, rather than hatching from eggs. Aphid nymphs pass through 4 (or in a few cases 3) instars. Mature aphids do not moult.
From The Amateur Entomologists' Society Glossary (Feb. 2016)
The first image below shows a young nymph of Symydobius oblongus (Shiny birch aphid) mid-way through her moult. The second image shows a similar nymph completing her moult.
Since insects generally undergo a number of moults prior to adulthood it must be important to them. Nontheless, moulting (=ecdysis) has downsides. Moulting is an expensive, risky and exhausting business for insects. Casting-off their skins (as exuviae) costs precious materials and energy, it immobilizes the insect, can result in serious injury, and renders insects soft & vulnerable immediately thereafter.
The colour variation among these (wingless) nymphs and (winged) adult Drepanosiphum platanoidis (common sycamore aphids) are because many moulted recently.
Air-breathing insects face a further difficulty. During moulting, respiration is seriously limited because their trachae are blocked whilst the linings are pulled out (Camp, et al., 2014).
So why moult?There are several reasons why insects, like other arthropods, moult.
Furthermore, since aphid exuviae smell like aphids, Muratori et. al (2008) found they can act as decoys for parasitoids. So what is meant by complete and incomplete metamorphosis?
Mayflies (=Ephemoptera), another 'ancient' insect group, do not pupate. Shown below, an adult mayfly completes its final nymphal moult, leaving behind the exuvia. Adult mayflies do not feed, they disperse, mate, and reproduce.
Below are the eggs, young nymphs, and an adult Cinara pinea (large pine aphid).
Having different body forms enables juveniles and adults to specialize in different tasks, to occupy different niches - and to avoid different predators. Nymphs often have a quite different defensive strategy to the parents. The adults of Cinara confinis have strong dark green aposematic (warning) coloration to discourage predators. Their nymphs tend to cluster together and rely upon their cryptic coloration for concealment. Pterocomma pilosum, (hairy bark aphid) nymphs are wonderfully cryptic, but their wingless adults are strongly striped and very conspicuous - perhaps because it takes time for the aphid to sequester enough chemicals from their host-plant to deter would-be predators.
Metamorphosis is, of course, an ancient trait among arthropods.
Holometabolous insects arose from hemimetabolous insects monophyletically, around 285 million years ago, by retaining (or repeating) embryonic features in the juvenile stages (Erezyilmaz, 2006). Holometabolous larvae are often grouped according to how they resemble embryonic segmental development stages (Richards, 2013), but holometabolous larvae are not merely prematurely hatched embryos. Holometabolous & hemimetabolous insect embryos both undergo three moults (known as cuticular depositions) - and holometabolous larval forms have evolved to serve their current lifestyles.
For insects that undergo incomplete metamorphosis, young nymphs, mature nymphs and adults can still look quite different. The images below show a group of first-instar nymphs, a second instar with its cast-off cuticle, a third, and fourth instar, and a wingless, parthenogenetic adult Macrosiphum euphorbiae (Potato aphid) - with her first-instar nymphs.
Below are fourth-instar nymphs of Lachnus roboris (variegated oak aphid), attended by ants, and a winged vivaparous adult. In this species vivaparous adults may be winged or wingless. Wingless morphs are produced by supressing wing development (Braendle, et al. 2006), but only nymphs destined to become winged adults have visible wing-buds. The fourth-instar nymph wing buds only become wings after moulting to adult.
Immature oviparae may be noticeably spindle-shaped, whereas fourth-instar nymphs destined to become alates have wing buds and longer antennae. The first image below shows developing and adult oviparae of Rhopalosiphum padi (bird cherry aphid). The red aphids in the second image are immature males of Macrosiphoniella millefolii (yarrow aphid) - in this species only the males are red.
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