In recent years, people are gaining more interest in animals vulnerable to extinction. I bet that when you read the previous sentence, your mind immediately pictured a rhino or an elephant, perhaps a cute panda or a gorilla, or maybe a whale if you are attached to the ocean. Don't feel bad about that. It also happened to me, a biologist in love with insects. Even when I'm conscious of this bias all the time, it is difficult to decouple the thought of extinction with mammals.
For me, this mindset changed a lot during a fieldwork trip. It was a warm sunny day in Bavaria, Germany. Since I moved from the other side of the Atlantic, my knowledge of local fauna is still building up (which translates to I have no clue about 80% of the things I see). I was focused on finding grasshoppers when I heard my colleagues excited about a finding. There it was: a bright white butterfly, with details in black and striking red spots. Even for a relatively big butterfly (~8 cm), it was flying slowly and graciously from flower to flower, allowing us to get closer for some pictures. Even for a group of insect-nerds, the excitement on the air was abnormal.
It is an Apollo Butterfly - said my supervisor.
And? What is special about it? - I made obvious my ignorance.
It is rare to see. It is considered as Vulnerable - he clarified. 
The responsible institution to assign these categories is the IUCN. In the IUCN Red List they gather all the available evidence and asses the risk of extinction to each species. It's not common to think about insects threatened to be extinct, and we should consider that it is also hard to get a good estimation of their populations for the majority of them. In fact, on the Red List there is information of 1051 species of Lepidoptera (butterflies and moths), when this group has at least 160.000 described species.
For the Apollo butterfly (Parnassius apollo) the first reports of population decline and extinction in Europe occurred already at the beginning of the 20th century. The causes of their decline are related to agriculture and urbanization, which reduces their habitats all over Europe. Besides these, their decline can also be aggravated by threats like air pollution, climate change, and butterfly collectors. Because of the latter, this butterfly became the first invertebrate to be protected under CITES, meaning that international trading operations would be monitored and regulated in order to protect it from over-exploitation.
After this encounter, my mind still pictures the charismatic mammals when I talk about extinction. But now there are also birds perched in fantastic trees and amphibians in small ponds. Now they are all surrounded by multitudes of insects dancing all over, graciously as the Apollo butterfly.
📑 References and further info:
Gil-T F. (2016). "Distribución, actualizada y corregida, y estado de conservación de cuatro subespecies de Parnassius apollo (Linnaeus, 1758) (Lepidoptera, Papilionidae) en el tercio sur de la Península Ibérica (E Andalucía, S España)". Arquivos Entomolóxicos 16: 203-210.
Mira Ó, Sánchez-Prieto C.B, Dawson D.A, et al. (2017). "Parnassius apollo nevadensis: identification of recent population structure and source–sink dynamics". Conservation Genetics. ISSN 1566-0621
Nakonieczny M, Kędziorski A, & Michalczyk K. (2007). "Apollo Butterfly (Parnassius apollo L.) in Europe – its History, Decline and Perspectives of Conservation". Functional Ecosystems and Communities1(1): 56-79.
Lepidoptera Barcode of Life http://lepbarcoding.org/
IUCN Red List www.iucnredlist.org/species/16249/5593483
📸 Pictures: Irina Formoso.
Apollo butterfly (Parnassius apollo). Bayern, Germany. 2018.