Question The Bone Collector Caterpillar

‘Hawaiian caterpillar patrols spiderwebs camouflaged in insect prey’s body parts’

By Daniel Rubinoff, Michael San Jose, and Camiel Doorenweerd

Published 4th of April, 2025

Science 388, 428-430 (2025)

From the Research Article:

Abstract:
‘Lepidoptera is the most herbivorous of all the insect orders, with predatory caterpillars globally comprising less than 0.13% of the nearly 200,000 moth and butterfly species. Here, we report a species in which caterpillars are carnivorous inhabitants of spider’s webs, feeding on the arthropods that they find there. This Hawaiian lineage also boasts an unprecedented and macabre practice of decorating its portable larval home with the body parts of the spider prey it harvests from the web where it resides. Phylogenomic data suggest that the origin of this unique spider cohabitant is at least six million years old, more than one million years older than Hawaii’s current high islands. After decades of searching, only one species has been discovered, and it is restricted to 15 square kilometers of a single mountain range on the island of Oʻahu, meaning that other members of the lineage have disappeared from older islands. Conservation action to save this globally unique lineage is imperative and overdue.’

From the Introduction:

‘These newly discovered caterpillars are the first known to depend on spider webs, using only those located in tree hollows, logs, or rock cavities and never leaving their immediate vicinity. Carnivorous caterpillars are an extremely rare evolutionary phenomenon, and although caterpillars and spiders are common in the same environments all over the world, only this single caterpillar lineage in Hawaii is known to have made the leap to spider cohabitation.’

‘Bone collector caterpillars crawl through the jumble of web and detritus, and opportunistically eat any weakened or recently deceased insects they come across (e.g., cached spider prey), even chewing through silk to reach their meal if need be. Because they exclusively rely on cobwebs in enclosed spaces (not sheet webs), they can access the full three-dimensional space of the webs. We have identified body parts belonging to more than six different families of insect attached to the silk caterpillar cases, suggesting that they are adaptable scavengers and predators.’

‘When decorating their silken portable cases, the caterpillars are particular. Body parts are carefully measured for size before the caterpillar weaves them into its collection. Each prospective new addition is rotated and probed with its mandibles several times, and parts that are too large are chewed down to a size that will fit its case. If denied access to arthropod body parts in captivity, the caterpillars do not accept other bits of detritus, suggesting that they recognize and exclusively use corpses in nature and that this decoration is important to their survival.’

‘When decorating their silken portable cases, the caterpillars are particular. Body parts are carefully measured for size before the caterpillar weaves them into its collection. Each prospective new addition is rotated and probed with its mandibles several times, and parts that are too large are chewed down to a size that will fit its case. If denied access to arthropod body parts in captivity, the caterpillars do not accept other bits of detritus, suggesting that they recognize and exclusively use corpses in nature and that this decoration is important to their survival.’

Ancient Origins:
‘The bone collector caterpillar belongs to Hyposmocoma, an endemic genus of small moths that is one of the most ecologically diverse adaptive radiations on the planet and, at 14 million years old, one of Hawaii’s oldest (7). The genus contains >350 species occurring from the splash zones of the tropical shorelines to frigid alpine deserts on volcanic slopes >3200 m high, with each species typically restricted to a part of a single volcano on a single island. As shown by phylogenomic inference from 38 loci and subsequent molecular dating, there are nine major lineages of Hyposmocoma, most between 9 and 15 million years old, far older than the oldest current high island of Kauaʻi (Fig. 3Opens in image viewer). The bone collector species is the only one known of its kind, representing a monotypic lineage without a sister species. Although it is related to the other carnivorous lineage of Hyposmocoma, their ancestors diverged more than 5 million years ago.’

Uncertain Future:
‘Although the bone collector species is able to use non-native spider hosts, it is still rarely found and its range is limited to a small area on one mountain on a single island. Population numbers may not be stable, and many factors leading to native insect decline in Hawaii (10) may also be affecting it, including introduced predators such as ants and parasitic wasps. It is unclear when bone collector caterpillars may have disappeared from Kauaʻi, but it was before they could be discovered and may have been due to anthropogenic causes, as has been the case with most of Hawaii’s historic extinctions [e.g., (16)]. Without conservation attention, it is likely that the last living representative of this lineage of carnivorous, body part–collecting caterpillars that has adapted to a precarious existence among spider webs will disappear.’

Figure 1: Pinned adult female (left) of the bone collector caterpillar and portable case (right) in which the larva resides decorated with body parts from ants, bark beetles, weevils, and flies.

Figure 2: Rotting wood log broken open to expose a bone collector caterpillar resting on a clump of webbing next to a non-native spitting spider (Scytodes sp.) with its egg sac. The web is partially obscured by termite and other wood-boring insect frass.

Reference:
Daniel Rubinoff et al. Hawaiian caterpillar patrols spiderwebs camouflaged in insect prey’s body parts. Science 388,428-430(2025).DOI:10.1126/science.ads4243