Matt Hayes, Research Assistant on the Butterflies through Time project, writes:
This insect is one of my favourite specimens on display in the Museum of Zoology and its common name perfectly captures its appearance – it really does look as if it has a peanut for a head! For me this is a reminder of the truly weird and wonderful forms the natural world can ‘dream up’, especially, I find, when considering insects.
However, despite the name at first appearing incredibly apt, it is nearly entirely wrong: the ‘peanut’ is not strictly the insect’s head, but is instead an elongated structure that branches off from it; the species is not a fly but is actually a type of ‘true bug’ and it does not really light up, as its name ‘lantern’ would suggest. This is all rather strange, so what is going on here?
Why does it have a peanut on top of its head?
This species belongs to a group of insects called lanternflies, family Fulgoridae, many of which also have elongate bulbous structures branching off from the top of their head, in different shapes and sizes.
Often in nature, a peculiar form can be explained by sexual selection, where the female prefers and chooses to mate with males that possess elaborate colours or large ornaments. This ‘selection’ allows the genes for these strange and striking forms to be passed on, without them offering a benefit to the survival of the individual. However, this usually results in each sex looking different, with females lacking the colours and ornamentation of the males. Both sexes of the peanut-headed lanternfly possess the ‘peanut’, meaning that another explanation is needed.
A popular and plausible suggestion is that the structure acts as a form of defence mechanism. Although it looks like a peanut from the top, if you look at a side view, and really use your imagination, it begins to look a little more like the head of a reptile, such as a snake or lizard, complete with false eye spots and teeth. This gives rise to one of its other common names, the ‘Alligator Bug’. Although perhaps unconvincing to us, it could be that a would-be predator is tricked into thinking the harmless lanternfly is actually a larger vertebrate, which could lash out, bite or otherwise cause it harm, and is therefore best to leave alone.
Another related suggestion is that the elongate structures of lanternflies distract predators away from the true head, something that is also aided by the presence of large eye spots on the edges of the peanut-headed lanternfly’s wings. If you are a predator, it usually pays to go for the head as you are more likely to make a killing blow. In most species, the head is pretty reliably located at the tip of the animal’s front end (anterior) and it tends to be the part of the body that possesses eyes. Therefore, a predator may be tricked into striking at the bulbous tip of the lanternfly, instead of the true head just below it, or at the eye spots on its wing edges. The lanternfly may still suffer a nasty nip, but it is far more likely to leave with its head, which all in all is definitely a win.
Others have put forward the idea that the hollow structure might be involved with sound resonance, creating, amplifying or detecting acoustic signals, but all of these theories warrant further testing, so that the ‘peanut’s’ true function can be ascertained.
What is a ‘true bug’?
Although in everyday language people often use the word ‘bug’ to refer to any kind insect or minibeast, the term should really only be used to refer to insects belonging to the order Hemiptera. In other words, all bugs (hemipterans) are insects, but not all insects are bugs.
The Latin name for this group translates into half – wing (Hemi – ptera), due to the fact that many species have forewings that are half transparent and half opaque, with the opaque half reinforced to look more like the wing cases of a beetle. All members of the group have a straw-like sucking and piercing mouthpart. Most species, such as the lanternflies, use this to drink the sap out of plants, but others, like assassin bugs, use it to pierce and drink the insides of other insects! More familiar hemipterans in the UK include aphids and shield bugs, which are typically in the region of 1 – 15mm in length. Lanternflies, which are mainly found in the Americas, but also Africa and Southeast Asia, are much larger relatives, with the peanut-headed lanternfly growing to have a wingspan as large as 15cm.
Why is it called a lanternfly if it doesn’t actually light up?
This strange myth seems to stem from an early description of lanternflies from 1705 when a German artist-naturalist Maria Sybilla Merian published a book called ‘Metamorphosis Insectorum Surinamensis’. She wrote that the head of a lanternfly can light up at night when both sexes are present, and that the light is bright enough to read by. Since then, scientists have questioned this finding and it has now been pretty comprehensively falsified. In 1983, B. W. Ridout tested all known bioluminescent processes on the ‘peanut’ of the lanternfly and did not get a response from any of them. Why Merian would make this false claim is unclear but it is possible that she confused the lanternfly with a bioluminescent click beetle that can be found in the same region. Therefore, although we now know that they do not light up, the name has stuck, and lanternflies will likely forever be known for a behaviour they never exhibited.
Lighting up at night is not the only myth these insects have inspired. In Costa Rica, the peanut-headed lanternfly is feared on account of locals believing that the ‘peanut’ is full of poison. Some believe that if a person is bitten it can quickly lead to their death and that the only cure is to have sexual intercourse! The exact urgency of treatment varies depending on who is asked, with some suggesting the antidote is required within 24 hours, whilst others report that it is required almost instantly, within 15 minutes of the bite taking place. The species is actually harmless, so evidently something about the strange appearance of the peanut-headed lanternfly causes impressive legends to be conjured up around it.
For all of the reasons I list above (the amazing resemblance to a peanut; the fascinating potential functions of this strange structure; the fact that the species is an example of a true ‘bug’ and the myths, legends and falsehoods it has inspired) this species has always peaked my interest. I hope this animal byte has helped shed some light on its weird and wonderful ways.
For more information please see:
The Fulgoridae (Hemiptera, Fulgoromorpha) of Guatemala. G. Goemans
Animal Bytes 
Fulgora lanternaria in Costa Rica appears when wings are wide spread, to have two large eye spots that may enhance sign stimulus to promote mating, and to give a defensive advantage in aposematic behavior and causing fright
posturing deterring predators. Tor Hansen