How to wire multiple ir photodiodes on one arduino pin?
I'm afraid that doing it the simple way simply will not work. Let's say that your diodes are drawing 1 mA each. With 50 in parallel, that will be a total of 50 mA. Assuming a resistor of about 50 ohms, your nominal voltage will be $$V= 5 - iR = 5 - .05\times 50 = 5 - 2.5 = 2.5 \text{ volts} $$ and if a bug blocks all of the light to one diode the current will drop by 1 mA, and the voltage will increase by $$\Delta V = .001\times 50 = 50\text{ millivolts} $$ While this is detectable with an Arduino, it overlooks something important. If the overall illumination level changes by 2%, the current will also change by 1 mA, and the voltage change will be impossible to distinguish from a bug.

What you need to do is to sample each diode in series, and fast enough not to miss a bug, and at your apparent level of knowledge I haven't the faintest idea how to do that. 

Sorry.