What makes a happy face? Determinants of colour pattern in the Hawaiian happy face spider Theridion grallator (Araneae, Theridiidae)

Theridion grallator, a small (<5 mm) spider endemic to wet and mesic forests of Hawaii, is highly polymorphic in its abdominal colour pattern. Morphs were either translucent yellow (the background colour only; no pattern) or exhibited superimposed pigmentation. These latter, patterned morphs were grouped into eight major categories, including opaque white and various configurations of red, black or maroon. The unpatterned morph was the most morph common among immatures (69 per cent), mature males (62 per cent) and mature females (62 per cent). All spiderlings were initially unpatterned; patterns developed at 10 to >100 days after hatching. Time required for completion of pattern development was roughly proportional to the amount of red in the final pattern. Mature males and females had similar morph frequencies overall, yet three rare morphs were seen only in females. Phenotypic frequencies of families were consistent with Mendelian inheritance at one locus, with the allele dictating absence of pattern recessive to alleles for patterned morphs.