The snout-vent length is 48.2 to 56.4 mm in adult males from Costa Rica and 60.2 to 71.8 mm in adult females from Costa Rica (Duellman 2001). Panamanian individuals are larger, with males measuring 67.6-75.6 mm and females 81.6 to 86.7 mm, and one female from Colombia measuring 92.8 mm (Duellman 2001). The head is as broad as it is long, with a snout that is subelliptical when viewed from above. The eyes are large, with reticulated lower eyelids. The diameter of the tympanum is equal to about one-half diameter of the eye (Savage 2002). The tympanum is moderately distinct, with the upper and posterior edges covered by a dermal fold (Duellman 2001) This dermal fold runs from the posterior of the eye over the tympanum, and continues as a flap to the axilla (Duellman 2001). The body is thin (Duellman 2001). Dorsally the skin is smooth, as are the ventral surfaces of the limb, while the belly and proximal ventral surfaces of the thigh are granular (Duellman 2001). Upper arms are slender but forearms are robust, with a noticeable dermal fold running across the elbow and along the ventrolateral edge of the forearm down to the disc of finger IV (Duellman 2001). The fingers and toes have discs (Duellman 2001). Fingers are about 3/4 webbed; toes are fully webbed in Panamanian specimens but slightly less so in Costa Rican specimens (Duellman 2001). Savage (2002) reported the webbing formula for fingers as I 2--2 II 1-1 ¾ III 1-1 IV and for toes as I 1-2- II 1-2- III 1-1 ½ IV 1 ½ - IV. There are flattened, bifid subarticular tubercles under fingers III and IV (Duellman 2001). The thenar tubercle is greatly enlarged but the palmar tubercle is not present. On the hindlimbs, skin on the shank tends to form a thin longitudinal fold on the ventrolateral and ventromedian surfaces, and a thin dermal fold extends across the heel and along the outer edge of the tarsus to the base of the disc on the fifth toe (Duellman 2001). There is a strong tarsal fold extending the entire length of the tarsus (Duellman 2001). On the posteroventral surfaces of the thighs, a distinct row of granules is present (Duellman 2001). There are many plantar tubercles, and supernumerary tubercles on proximal segments of the toes (Duellman 2001). The inner metatarsal tubercle is low and elliptical, and there is no outer metatarsal tubercle. In adult males, there are spiny, brown nuptial pads on the base of the thumb, as well as short, paired vocal slits and a slightly distensible single median subgular vocal sac (Savage 2002).
The color of the frog changes from day to night (metachrosis). During the day, dorsal surfaces of the head, body, limbs, fourth fingers, and fourth and fifth toes are light, yellowy green, turning darker green at night. Dorsal surfaces often have black-bordered white, pustular spots; this species is the only Agalychnis species in which the white spots may have black outlines, and the black borders seem to be present in all Panamanian and Colombian specimens but lacking in some Costa Rican specimens (Duellman 2001). The upper surfaces of the forearms, flanks, belly, anterior and posterior thigh surfaces, webbing, and most digits are orange (Duellman 2001). Individuals from Barro Colorado Island, Panama, had a yellow chin and throat, while those from Tacarcuna, Panama had a creamy white throat and chest, and those from the Peninsula de Osa, Costa Rica, had a cream throat the chest and a pale orange belly (Duellman 2001). In some individuals there is a dark green stripe separating the green dorsum from the orange flanks (Duellman 2001). Yellowish cream stripes run from the elbow to the disc of finger IV, along the edge of the tarsus from the heel to the disc of toe V, and across the anal flap (Duellman 2001). The iris is dark red, and the palpebral reticulations are greenish gold. Juveniles are green during the day but reddish brown at night (Savage 2002).
The length of the larva is 41 mm at stage 35. The body is robust and the tail is moderate, its tip tapering to a thin flagellum. Eyes are located dorsolaterally and directed laterally. Nares are dorsolateral, directed anteriorly (Savage 2002). The mouth is anteroventral, with a small and entire oral disc that has beaks and 1/3 rows of denticles (Savage 2002). Both beaks are serrated, and there are two or three rows of papillae lateral and ventral to the mouth. The spiracle is lateroventral and sinistral, and the vent is dextral. The body coloration is bluish gray with dark brown flecks on the sides and the anterior portion of the fins. The caudal musculature is reticulated brown and gray anteriorly (Savage 2002).
The species was first described by Boulenger (1913). The name given is a patronym for Dr. H. G. F. Spurrell, who collected the type series in Colombia (Duellman 2001). Roberts (1994a,b) pointed out similarities between this species and A. saltator in breeding behavior and locomotion.
A Spanish-language species account can be found at the website of Instituto Nacional de Biodiversidad (INBio).
- Boulenger, G. A. (1913). ''A collection of batrachians and reptiles made by Dr. H.G.F. Spurrel, F.Z.S., in the Choco, Colombia.'' Proceedings of the Zoological Society of London, 1913(4), 1019-1038.
- Duellman, W. E. (2001). The Hylid Frogs of Middle America. Society for the Study of Amphibians and Reptiles, Ithaca, New York.
- Roberts, W. E. (1994). ''Explosive breeding aggregations and parachuting in a neotropical frog, Agalychnis saltator (Hylidae).'' Journal of Herpetology, 28(2), 193-199.
- Roberts, W.E. (1994). ''Evolution and ecology of arboreal egg-laying frogs.'' Ph.D. dissertation, University of California, Berkeley
- Savage, J. M. (2002). The Amphibians and Reptiles of Costa Rica. University of Chicago Press, Chicago and London.
- Scott, N.J. and Starrett, A. (1974). ''An unusual breeding aggregation of frogs, with notes on the ecology of Agalychnis spurrelli (Anura: Hylidae).'' Bulletin of the Southern California Academy of Sciences, 73(2), 86-94.