Ectopic expression of odd^hs.PS in embryos using heat shock can result in segmental defects, with defects occurring with the highest penetrance when heat pulses are administered at stages 5-7. Three major segmental phenotypes are seen; firstly a phenotype with head defects only, secondly a pair-rule phenotype and thirdly a pair-rule phenotype restricted to the dorsal half of the embryo. The head defect phenotype is the earliest phenotype seen and consists of specific loss of head regions beginning just anterior to the T1 beard up to the maxillary sense organs. The missing structures are replaced by a cone-shaped indentation. Central portions of the mouthhooks, including the ectostomal sclerite, are malformed or missing. Alternate segments are deleted in the pair-rule phenotype, the phasing of the deleted regions is neither segmental nor parasegmental. Composite denticle belts are seen on the ventral side of the embryo with the regions in between deleted. The most frequently fused segments are A4 and A5, with sensitivity then decreasing away from A4/A5 towards the extremities of the embryo (A6/A7 >= A3/A4 > T3/A1 > T1/T2).