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Insects are renowned for their strange tastes in food, but few rival
Lobocraspis griseifusa, a moth from Southeast Asia. To find it, you must
go to the forest, wait until nightfall, then look into the eyes of an amenable
water buffalo. If your luck holds, you will witness a jostling crowd of
moths intent on a single delicacy: tears.

When researchers first announced that certain moths drank tears, their
reports were greeted with scepticism. Some entomologists thought it a freak
behaviour, or suspected that the insects involved were flies rather than
moths. Yet subsequent research has revealed tear drinking to be a specialised
and sophisticated strategy, with important medical and veterinary implications
.

Moths that drink tears are fastidious in their tastes, restricting their
attentions to certain species of animal. The usual victims are either hoofed
mammals, such as cattle and other bovids, deer, horses, tapirs and pigs,
or elephants – and, on occasion, people. Researchers have yet to witness
moths visiting the eyes of carnivores, marsupials, birds, or members of
any other group of vertebrates. This preference for certain hosts is not
yet fully understood, but it may reflect differences in the chemical composition
of tears from different animals. Another factor could be the host’s behaviour.
The most frequent victims are the most placid and tolerant mammals – an
important consideration for insects that habitually fumble around the eyes
of large animals with intent to steal.

Moths adopt a variety of ingenious techniques in their quest for tears.
Lobocraspis griseifusa, which is one of the most highly evolved tear suckers,
irritates the eye of its victim, thereby inducing a copious flow of tears.
It repeatedly seeps its proboscis across the host’s eyebal at irregular
intervals, desisting if the host shows signs of retaliation. L griseifusa
is capable of intruding its proboscis between the lids of a closed eye,
so it may continue to feed when its host sleeps.

Other species are less subtle; they can feed only on the most insensible
of apathetic hosts. A moth of the genus Poncetia, for example, has such
a short proboscis that it is forced to cling very close to, or directly
onto, the eyelid of its victim. The victim reacts by pressing its eyelids
together and squeezing its tormentor.

The moth attempts to extricate itself, but, if successful, it does not
fly off to safety. Instead it returns for more, sucking vigorously – almost
as if inebriated. When I discovered this species last year, I gave it the
scientific name lacrimisaddicta (the ‘tear addicted’) to commemorate its
greedy manners.

Other species are of a more gentle disposition and probably cause no
more than a tickling sensation. Some are so tactful in their approach that
the host does not realise that its eye is under attack. Filodes mirificalis,
for example, has a large wingspan and so is more conspicuous than many other
species, but it has a slender body, elongated legs and an exceedingly long,
thin proboscis. It lands softly on the host at some distance from the sensitive
eye region and then applies its proboscis in a most discreet fashion.

Another weapon in the armoury of some moths is persistence in the face
of disturbance. The behaviour of Chaeopsestis ludovicae provides a striking
illustration. If photographed with a flash gun while feeding, this sensitive
moth falls from its host. However, before reaching the ground it recovers
and flies up again – often landing on the eye that it has just left. On
one occasion I made five successive exposures, each prompting the moth to
fall from, and fly back to, the eye of a zebu. A similar series of eight
rounds followed when the moth settled on a second zebu.

Behavioural tactics are not the only requirement for animals that make
a living by absconding with valuable body fluids. The most successful operators
are usually the smallest. Mabra elephantophila (named for its association
with elephants) is a case in point. With a body mass less than that of a
face fly, this shy, delicate moth is among the smallest tear-loving species.
It is successful in gathering tears, not because of any special behavioural
trick, but because its diminutive size renders it hard to see or feel. At
the opposite extreme is Tarsolepsis remicauda, a bulky moth with a wingspan
of 80 milimetres. On the forewings are two silver triangles with a metallic
shine – a rare decoration in moths. Two brushes of very long bright red
hairs adorn the base of the abdomen. This combination of size and flamboyance
make this moth one of the least successful tear drinkers.

As these examples show, the pursuit of tears takes a variety of different
forms. It is not the exclusive preserve of a single assemblage of moths,
but occurs in several rather distantly related families: the Noctuidae,
Notodontidae, Geometridae, Pyralidae, Sphingidae and Thyatiridae. The habit
has, it seems, evolved on more than one occasion.

What do moths gain by sucking at such unlikely places as buffalo or
elephant eyes? The fact that tears contain salt could be one of the main
attractions. Many other creatures, from butterflies to mammals, resort to
special sources of salt – such as salt licks – when the need arises. For
moths, tears may meet a similar need. Another motive could be the search
for water, especially in dry regions or seasons, but this cannot be the
only explanation. Moths seek, and avidly imbibe, tears near a stream or
pool, or when there is dew and even while rain is falling. Furthermore,
several species suck for long periods and take more than their fill of water,
expelling the surplus through the anus.

Tears also contain another useful commodity: protein. Regular tears
are a good source of albumin and globulin – and diseased eyes yield an extra
harvest of epithelial cells and while blood cells. Can moths digest and
make use of this food source? Until recently many entomologists would have
answered in the negative, because moths generally subsist on sugars, not
proteins. Yet tests have confirmed that the digestive powers of some tear
moths are exceptional. I have found that at least three species – Lobocraspis
griseifusa and two species of Arcyophora – can break down proteins in their
food. Less specialised forms, such as Filodes mirificalis, cannot digest
proteins, so they are probably gathering tears as a source of salt.

Moths that exploit tears are able to draw upon a source of food that
is both constantly available and of high quality. Yet, for most species,
tears form only a part of a much broader diet, which includes other body
fluids, such as saliva, nasal mucus, skin secretions, the tiny blood droplets
exuded anally by mosquitoes, serum and blood from wounds, as well as urine
and dung. Several species suck from skin sores on the underparts of elephants.
Many moths that enjoy a diet of mammalian body fluids confine their attention
to secretions smeared on vegetation or left on the ground. A substantial,
but smaller, number take these fluids direct from the bodies of mammals.
A still smaller group feed on tears as well as on other fluids. Fewer than
10 species feed exclusively on tears.

A special snack in the dry season

Those moths with broad feeding habits tend to alter their diet with
the changing seasons, shifting from tears to other body fluids during the
rainy season. As this pattern implies, most attacks on human eyes take place
in the dry season – although humans remain somewhat unusual hosts. In my
experience, a long spell without attacks may be followed by several during
a short period, or even during a single night. Although some moths suck
only perspiration, a dozen species have repeatedly feasted on my tears,
the most common being Pydnella rosacea and Pionea aureolalis. The sensation
of having one’s eyes sucked is somewhat unpleasant, rather like having a
grain of sand on the eyeball. People would normally flip off any such intruder
at once. But if asleep or preoccupied, they may be unaware that their tears
are being stolen, especially if the thief is one of the more gentle moths.

Moths that drink tears occupy a vast geographical range, which includes
the tropical belt of Africa, Asia and America and some adjacent subtropical
regions. Careful searches have failed to uncover them in Europe. Forest
is their preferred habitat, but many venture into open pasture some kilometres
beyond the forest fringe. The savanna is home to a few species, including
some of the most advanced. Tear moths live at altitudes ranging from sea
level to 2100 metres.

Tear drinking is most highly developed in regions in which a dry season
alternates with a humid one. In monsoonal Thailand, for example, nearly
100 species of moths from six families drink tears. By contrast, areas in
which humid weather persists all year tend to be home to fewer, less advanced
species. In permanently humid western Malaysia, only some 20 species from
four families have so far been observed – although this number may rise
as further field work is carried out.

Research on tear moths has a habit of producing surprises. Take the
strange case of Pionea on the island of New Guinea. New Guinea was originally
thought to be without tear moths, simply because suitable hosts are not
native to the island. Although pigs arrived there several thousand years
ago, the most attractive hosts, such as deer, cattle and water buffalo,
were introduced only during the past few centuries, leaving little time
for moths to evolve the habit of taking tears.

Yet when I went there in 1982, I discovered several specimens of a single
species, Pionea damastesalis, in the act of imbibing tears from water buffalo
and zebu P damastesalis is not an endemic species. On the contrary, its
range stretches from Sri Lanka and India, through Southeast Asia, to Australia,
where it doubtless feeds at eyes just as it does in Asia. Presumably the
species reached New Guinea as a fully fledged tear drinker, either relatively
recently or when the first acceptable hosts arrived there. People may have
introduced the species inadvertently with agricultural crops. Whatever the
exact details, the colonisation of New Guinea proves one thing beyond doubt:
the pursuit of tears is a highly effective way of making a living.

* * *

A legacy of sore eyes

Moths that drink tears are suspected of transmitting eye diseases in
cattle and water buffalo. One such disease is conjunctivitis – inflammation
of the eye’s transparent outer membrane – which can lead to temporary, or
permanent, blindness. Conjunctivitis in bovids can be caused by mechanical
or chemical irritation, or by infection with microorganisms or nematodes.
Insects that flit around eyes are obvious suspects in the chain of transmission.
Flies, notably the face fly (Musca autumnalis), have been incriminated as
vectors of Moraxella bovis, the bacillus most frequently associated with
the disorder in North America. Yet the moths Lobocraspis griseifusa and
species of Arcyophora could also transmit the diesease. Their proboscises
are applied more aggressively than those of face flies and they bear structures
which can probably inflict microscopic lesions on the eyeball and eyelid.

At one time, biologists thought that L griseifusa could pierce the conjunctiva
or skin near the eyeball and suck blood, but this notion has now been disproved.
Unlike the proboscis of some Calyptra moth species, which can pierce the
skin to suck blood, though never at eyes, the mouth parts of L griseifusa
are neither robust enough, nor sufficiently well armed, to penetrate any
tissue. Moreover, biologists have not observed the sort of behaviour that
usually accompanies piercing. Yet although tear moths evidently can neither
bite, nor cause macroscopic wounds to the conjunctiva, there is a real possibility
that they transmit disease-causing agents.

Dr Hans Banziger is in the department of entomology at Chiang Mai University
in Thailand.