This is a small (maybe 3/16 inch across the shoulders) jumping spider proudly exhibiting the fly it caught for an afternoon snack. How many flies does it take to grow a mature spider?
Factoid found on science trivia site: the weight of insects eaten by spiders each year is greater than the weight of the entire human population. There are 6,544,948,000 humans and counting. So if I knew the total weight of humanity, and the weight of an average fly, well then I could approximate the minimum number of flies eaten by all spiders in a year. The spiders I know of mature in a year or less. So, now if I knew how many spiders there are in the world, voila! a good guess at the number of flies consumed by the average spider in a year. Adjust that for the development time for the spider in question, and maybe its size relative to the average spider's size, and there's my answer!
How accurate do you think the factoid I started with is?