The life history of the harpactorine reduviid Sinea spinipes (Herrich-Schaeffer) was studied in southern Illinois from February 2001 to November 2002, and the immature stages were described. The bug also was reared from egg to adult under controlled laboratory conditions. This univoltine species occurred on vegetation along roadsides within forests, in open fields beneath or near scattered trees, and along forest edges. It overwinters as adults, which emerged in mid-April and were found through early September. Nymphs were found from early June through late September. The bug was reared in the laboratory on larval beet armyworm, Spodoptera exigua (Hübner), at 26 ± 3.0°C under a photoperiod of 16:8 (L:D) h. The incubation period averaged 15.13 d; eyespots appeared in ≈7 d. The five stadia averaged 8.15, 9.22, 9.21, 11.43, and 19.59 d, respectively. The total developmental period averaged 72.73 d. Instars can be distinguished by differences in several anatomical features, including body length and width and progressive development of size, number, and pattern of spines.