Nature Notes
by
Philip Radford
I expect that most of us are impressed by the large size of the eyes of dragonflies
which, for the bigger species, seem to make up much of the head, with
such a different appearance from that of a butterfly. Butterflies have long, clubbed
antennae and relatively small eyes, while the antennae of dragonflies are
short and often inconspicuous. In contrast, the antennae of moths are big and
often broad and feathery, with those of the males being much larger than those of
the females; because of these sensitive antennae, some male moths are able
to detect the presence of females , by scent, over sometim es quite
remarkable distances. This long-range sensitivity is invaluable for night-flying
moths where the population is low, at least if they are to have a chance of mating.
With certain moth species, the males fly by day while females only fly by night;
an example here is the handsome Emperor Moth, with big eye spots on the
wings, and which is found on the Quantock moors and on Exmoor. The small
male Emperors, with big bushy antennae, fly on sunny days and are attracted,
by scent, to females as they rest amongst heather.
The thin, clubbed antennae of butterflies help the males to contact females by
scent as well, although sight plays a part with these day-flying insects. Additionally,
butterflies detect suitable flower scents using their antennae, so that the
insect can have a feed of nectar; a butterfly feeds by inserting its proboscis,
really a long sucking tube, into the chosen flower. Another use of antennae in
butterflies is that, by scent, the correct plant can be detected on which the
female can lay her eggs; often foot movements help release scent from a leaf.
If eggs are laid on the wrong plant, then hatched caterpillars are unable to feed.
Moths, like butterflies, feed on flower nectar but, for some species, this is only by
night. However, one day-flying moth is the Humming-bird Hawk-moth and, as a
migrant from Mediterranean lands, it can be seen, remarkably, flying in the sun in
Quantock gardens; it is instructive to watch the hovering insect as it repeatedly
inserts its long proboscis into suitable flowers. Last July I saw one of these moths
which had evidently found a rich feeding source on a single red valerian, and
was probing its proboscis into one multiple flower after another, and all on the
same plant. The valerian was clearly ripe with tempting nectar; no doubt it was
the sense of smell which guided the moth; there were other valerian plants around
but they were all ignored
Vision must be very different for butterflies, moths or dragonflies than for ourselves,
although all the groups have colour vision of sorts. It seems that these
insects, broadly, can detect ultra-violet light and so recognise colour patterns on other insects, and flowers at times, which would not be recognised by people.
The three insect classes all have compound eyes, ranging from about five
thousand lens facets in good-sized butterflies to around twenty-five thousand in
large dragonflies. These compound eyes enable the insects to detect
movements around them, and to get a rough focus on nearby small objects.
Even so, this form of vision allows the insect to see an approaching enemy
and to react, either by flying off if perched or to take evasive action in the
air.
Anyway, returning to dragonflies with their very large compound eyes, these
insects feed only on other insects, which they catch in the air; flowers cannot
be of any interest to dragonflies, except as perches. Sometimes dragonflies
feed on other dragonflies, although not necessarily of the same species, and
butterflies are often victims for the larger types; obviously, much aerial agility is
required for successful hunting by dragonflies, and so much depends on vision.
Compound eyes essentially, have multiple facets, each having a lens on the
surface and a cone-shaped lens beneath: ligh t rays are directed to a
sensitive membrane, and the image is then relayed to the brain. The detailed
structure of such an eye is cer tai nly remark able. With the large
dragonflies , the eyes represent much of the head in size; in consequence,
the insect can sense movement around it in front, at the side
and even behind. In addition to these compound eyes, dragonflies have
three light-sensitive spots (ocelli), each with one lens, on the top of the
head; these simple eyes help the insect to keep stable in the air. The dragonflies'
big eyes and small antennae must indicate how these insects live.
Dragonflies catch their food and avoid enemies through vision; the short
antennae give some sense of smell and help the insect to feel, which
must be useful when eating its freshly-caught prey.