This research is an investigation of the mechanisms behind sexual dimorphisms in spiders. Specifically, my aim is to understanding why female spiders are typically larger than their male counterparts by answering the following three questions. 1. When does sexual size dimorphism (SSD) become noticeable in Parasteatoda tepidariorum’s (common house spider) development? 2. What are the molecular signals involved in SSD? 3. What is the evolutionary benefit of SSD?