Easter egg weevils

Fig. 1. Pachyrrhynchus speciosus (Pachyrhynchini) Udo Schmidt (CC BY-SA 2.0)
Philippines, Leyte, Hilusig, Mt. Balocaue https://www.flickr.com/photos/coleoptera-us/22079183651/in/photolist-zD4A5c-yWWqKs

Easter egg weevils, as they are sometimes called, are fabulously ornate and colourful insects in the tribe Pachyrynchini (Coleoptera, Curculionidae, Entiminae). They are flightless, plant-feeding curculionids found mainly in the Philippines (and a few islands off Taiwan). For some truly stunning photographs by French photographer Frank Deschandol, see here (and figures 1, 2 and 4).

Fig. 2. Pachyrrhynchus erichsoni (Pachyrhynchini) Udo Schmidt (CC BY-SA 2.0)
Philippines, Leyte, Hilusig, Mt. Balocaue https://www.flickr.com/photos/coleoptera-us/21625126664/in/photolist-dfiPJv-zD4A5c-yWWqKs-aW1eFR-aW1eYn-A8z4Pp/
Fig. 3. Map of the Philippines showing the location of all the regions and provinces
https://commons.m.wikimedia.org/wiki/File:Ph_regions_and_provinces.svg#

Alfred Russell Wallace (1891) was one of the first Europeans to observe these weevils:

In these islands also we find the extensive and wonderful genus of weevils (Pachyrhynchus), which in their brilliant metallic colouring surpass anything found in the whole eastern hemisphere, if not in the whole world.” Wallace (1891)

The tribe Pachyrynchini comprises of 18 genera, and about 600 species, of which about 83% are endemic to the Philippines (Cabras et al., 2022). For example, species in three genera, Pachyrhynchus Germar, Homalocyrtus Heller, and Metapocyrtus Heller (see below, Fig. 5), are present on the island of Mindanao, where they have been studied by the University of Mindanao Coleoptera Research Center (UMCRC), which has discovered a number of new species, as reported here. There are about 145 described species in the genus Pachyrhynchus, of which 93% of are endemic to the Philippines: the majority having a narrow geographic range, limited to forested habitats, either on a mountain range or an island.

Pachyrhynchus beetles have an astonishing variety of bright, iridescent elytral markings, which appear to function as aposematic signals. Lizards frequently attempt to feed on beetles, and other insects, but the colourful blue patterns of two pachyrhynchid weevils (Pachyrhynchus tobafolius and Kashotonus multipunctatus) were shown to deter predation by Swinhoe’s tree lizard (Tseng et al., 2014), shown below (Fig. 4).

Fig. 4. Swinhoe’s tree lizard (Japalura swinhonis) by leemt2 https://www.flickr.com/photos/leemt2/2544903068/in/photolist-oFumyV-oo2pKn-rHE2EG-c4GY1h-c48WS3-JQHwM-JQHsa-9TkfxZ-Rpv4yy-2c9EJaR-2c9EJ7p-2c9EJ48-PMdhS4-2c9EHZF-vraYGQ-vafWye-JQxXS-R6BX19-33RhY2-8hEAUy-oiSaF7-oiHc6m-fi3jEU-oiBGdP-o2pCpA-8fDyxV-8hEAxL-8jWFee-8jZFUC-8jZFZ1-8jWuNe-4UjdNS-4STi27-4STi43-4STi6f-8dEYwW-72zs1y

Lizards and other predators, soon learn to avoid these colourful weevils because of their extremely tough exoskeleton. Pachyrhynchus weevils, for example, have dome-shaped body armor, formed by firmly interlocked elytra, which reder them too hard to swallow! (Wang et al., 2018). The force required to crack open a weevil’s body was found to be significantly higher than the mean bite force of the local lizards (Wang et al., 2018). These ‘hard’ weevils are instantly spat out by the lizards, after they have bitten them.

Members of other weevil groups, including long-horned beetles in the genera Doliops and Paradoliops (Coleoptera, Cerambycidae) mimic the colours and appearance of Pachyrhynchus species, to send aposematic signals to ward off predators (see below, Fig. 7).

Fig. 5. Metapocyrtus elegans (Pachyrhynchini) Udo Schmidt (CC BY-SA 2.0)
Philippines, Leyte, Hilusig, Mt. Balocaue https://www.flickr.com/photos/coleoptera-us/21974187100/in/photostream/https://www.flickr.com/photos/coleoptera-us/21974187100/in/photostream/

Doliops beetles are also endemic to the Philippines and Taiwan and are represented by at least 66 species (65 from the Philippines, especially Luzon and Mindanao). They are often quite rare, with several species known only from the single type specimen, and most are listed as vulnerable. One particular diversity hotspot for both models (Pachyrhynchini) and mimics (Doliops longhorn beetles) is Mt. Apo Natural Park in Mindanao (Fig. 6, below). Another is  is the Mt Kitanglad Range Natural Park, in the Province of Bukidnon.

Fig. 6. Mt. Apo, Mindanao by Jay Jopia CC BY-SA 4.0 https://en.m.wikipedia.org/wiki/Mount_Apo#/media/File:Mount_Apo_Rainforest.jpg

A form of Batesean mimicry exists between species of the genus Doliops (Fig. 7, below) and members of the genera Pachyrrhynchus and Metapocyrtus (Curculionidae,  Pachyrrhynchini). The degree of mimicry is remarkable, according to Arvīds Barševskis (2013), because of its general similarity in appearance: the short legs, specific shape of antennae, colouration, and the shapes of the elytal spots and stripes. However, where the longhorn beetles benefit from imitating the model is that they are softer and more palatable than the pachyrrhynchid weevils. They are in effect, pretending to be harder than they are!

Fig. 7. Doliops johnvictori by Ben Sale https://www.flickr.com/photos/33398884@N03/16730905417/in/photolist-pdC6g5-qG8Ln7-qvyp2k-pj6weP-2n4HYEw-qfRoQa-2k8Qa1Q-po4zfe-iqXW2k-rusghv-iqXVAR-2kKGwJc-ALiWvU-AizkDJ-2kKCMPC-2kKGVXC-jXduTi-BgCgoe-BgCfyD-2nZEYmA

The existence of mimicry rings among the tribe Pachyrhynchini has been known since the time of A. R. Wallace (1889), but according to Analyn A. Cabras from the Coleoptera Research Center, Institute for Biodiversity and Environment, University of Mindanao, researchers have ‘barely scratched the surface of this topic considering so many mimics await discovery and description’ (Cabras et al., 2019). For example, a recently discovered form of Mullerian mimicry appears to exist between three species from the same location on Mindanao: Metapocyrtus kitangladensis sp. n.,  Coptorhynchus sp. and Pachyrhynchus cumingii (Cabras et al., 2019); all found in the same locality. This type of mimicry is considered as Mullerian, since all species share similar defense mechanism, i.e. the hardness of their elytra. They benefit from looking very similar, because predators that experience biting, and spitting out (!), one of them, will quickly learn to avoid all of them.

These flightless weevils are characterized by vividly coloured
patches of scales on their dark elytra (Fig. 8, below). These structural colours result from the reflection of differently oriented 3D photonic crystals within their scales (Chang et al., 2020).

Pachyrrhynchus orbifer Kohichiro Yoshida (CC BY-NC 2.0) https://www.flickr.com/photos/kohichiroh/8038189637/in/photostream/

Unfortunately, the beautiful colours of Easter egg weevils make them an object of desire for collectors: “the rampant illegal poaching of these species [Doliops and Pachyrynchus] as evident in different online stores selling specimens require immediate attention” (Cabras and Barševskis, 2016). Many are for sale on eBay, despite being listed as threatened or vulnerable. One piece of good news however, is that it has been possible to rear them in the laboratory, using their host plants (Huang et al., 2018).

Links

https://www.nationalgeographic.com/animals/article/jewel-weevils-philippines-beetles-discovery

Beetles, bugs, and other arthropods of the Philippine Tropics.  https://www.bakukangetal.com/2021/

Weevils & insects (Philippines-Taïwan-PNG) By Frank Deschandol:  https://www.flickr.com/photos/frank-deschandol/albums/72157712052576757

Doliops species (Philippines-Taïwan) By Frank Deschandol: https://www.flickr.com/photos/frank-deschandol/albums/72157713624972912

Dazzling new mimic beetles found, may already be under threat. https://www.nationalgeographic.com/animals/article/jewel-weevils-philippines-beetles-discovery?s=09

Inconceivable weevils (blog): https://wp.me/p2hWjJ-6S

https://www.calacademy.org/press/releases/new-study-reveals-how-easter-egg-weevils-got-their-spots

References

Barševskis, A. (2013). Contribution to the knowledge of the genus Doliops Waterhouse, 1841 (Coleoptera: Cerambycidae). Baltic Journal of Coleopterology13(2), 73-89.

Cabras, A., & Barševskis, A. (2016). Review on Doliops Waterhouse, 1841 (Coleoptera: Cerambycidae) of Mindanao, Philippines with description of a new species. Baltic Journal of Coleopterology16(2), 143-156.

Cabras, A. A., Medina, M. N. D., & Zhang, G. (2019). Metapocyrtus kitangladensis sp. n., a new Pachyrhynchuscumingii GR Waterhouse, 1841 mimic from Mindanao Island, Philippines. ZooKeys853, 119.

Cabras, A. A., Nique, G., Mohagan, A., Baden, J., Loevinsohn, M. E., Mugarura, J., … & Barnaud, C. (2016). Diversity and Distribution of Pachyrynchini (Coleoptera: Curculionidae: Entiminae) in Mt. Apo Natural Park, Philippines-JBES. Journal: Journal of Biodiversity and Environmental Sciences (JBES)8, 312-319.

Chang, Y., Ogawa, Y., Jacucci, G., Onelli, O. D., Tseng, H. Y., & Vignolini, S. (2020). Hereditary character of photonics structure in Pachyrhynchus sarcitis weevils: color changes via one generation hybridization. Advanced Optical Materials8(15), 2000432.

Huang, L. C., Huang, W. S., Lin, C. P., Nuñeza, O. M., Tseng, H. Y., & Tang, H. C. (2018). Captive breeding of two insular populations of Pachyrhynchus sarcitis (Coleoptera: Curculionidae) from Lanyu and Babuyan Islands. Journal of Asia-Pacific Entomology21(4), 1233-1238.

Tseng, H. Y., Lin, C. P., Hsu, J. Y., Pike, D. A., & Huang, W. S. (2014). The functional significance of aposematic signals: geographic variation in the responses of widespread lizard predators to colourful invertebrate prey. PLoS One9(3), e91777.

Wallace, A. R. (1889). Darwinism: an exposition of the theory of natural selection, with some of its applications. Macmillan and Company.

Wallace, A. R. (1891). Natural selection and tropical nature: Essays on descriptive and theoretical biology. Macmillan and Company.

Wang, L. Y., Huang, W. S., Tang, H. C., Huang, L. C., & Lin, C. P. (2018). Too hard to swallow: a secret secondary defence of an aposematic insect. Journal of Experimental Biology221(2), jeb172486.

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