Instructions
Purchase a bug net from Tom Nook's store. The net is not for sale every day, but when it is, the cost is always 500 Bells.
Search for the scorpion during summer nights. Scorpions only appear between the months of July and September and when it is dark, between 7:00 PM and 4:00 AM. Scorpions appear on the ground.
Locate the tarantula during summer nights. The tarantula only appears between the months of June and August and can only be seen at night, between 7:00 PM and 4:00 AM. The tarantula appears on the ground.
Find the emperor butterfly during summer afternoons. The emperor butterfly only appears between the months of June and September and only appears in the afternoons, between 12:00 PM and 4:00 PM.
Walk slowly up to an insect with your net ready so that you do not scare it off before you have a chance to catch it. When you are within range of your net, quickly tap the screen with your stylus or press the action button. If you are successful, you will catch the bug in your net, but if you are unsuccessful the insect will escape and you will have to find another.
Change the date of your Nintendo DS internal clock if it is not currently summer in your town. The time and day in your Animal Crossing town corresponds to that of the Nintendo DS clock. To change the date, go into the user configuration menu from the main menu of your Nintendo DS. Set the clock to July or August for best results as all three species of insects are available during those months.