Thecla Pseudolycaena damo (female)
 
Pseudolycaena marsyas, the Cambridge blue, giant hairstreak or Marsyas hairstreak, is a species of butterfly in the family Lycaenidae. Pseudolycaena marsyas has a wingspan of about 40–50 millimetres (1.6–2.0 in), a quite huge size in hairstreaks (hence the common name "giant hairstreak"). The uppersides of the wings are usually metallic blue, with hues varying from cobalt blue to turquoise blue depending on location. The apex of the wings are black and lightly falcade in males and the hindwings are tailed in both sexes, with a small black spot. The undersides of the wings are pale blue greyish, with several black spots and thin markings. This wide-ranging species occurs in Central and South America from Mexico up to Argentina, in semi-open forests at an elevation of about 0–1,000 metres (0–3,281 ft) above sea level.  Almost all neotropical Theclinae species are placed in the Eumaeini. The tribe is not particularly well represented in collections, so until fairly recently a high percentage remained unstudied, and were inappropriately filed away in the 'convenience' genus Thecla. Many taxonomists have attempted to rationalise the systematics of the Eumaeini, the most recent being Robbins who published a revision in 2004, reclassifying the taxa into 83 genera. Currently there are 1058 known species. Taking into account their small size, secretive behaviour, and the great similarities between many species, it is estimated that about another 200 species probably remain to be discovered.
The genus Pseudolycaena comprises of just 2 species - damo from Central America, and marsyas which occurs from Colombia to Paraguay, and is also found on several Caribbean islands.
Pseudolycaena marsyas is huge in comparison with most other hairstreaks, measuring almost 6cms in wingspan. The upper surface of the wings is metallic blue, although the hue varies according to location, from ultramarine in Caribbean forms, to turquoise-blue in Paraguay. In most forms the males have a falcate apex. All these forms are allocated to a single subspecies - Pseudolycaena marsyas marsyas. Both sexes are tailed.
Habitats
This species is found in disturbed semi-open forest habitats at altitudes between sea level and about 900m.