It has lately been found, however, that many Tachinids have much more complicated life-histories than this. I have already discussed some of the more important of these in my book entitled, "Seeing Nature First" ([pages 150-158]).
One can frequently rear parasites from the chrysalids of butterflies, but in many cases it is probable that these began their parasitic development in the caterpillars, which were able to change to chrysalids before being killed. In some cases, however, the chrysalids seem to be attacked, especially by certain Ichneumon flies.
Rearing Butterflies From Caterpillars
There are few things in the world more interesting to watch than the wonderful changes which a moth or butterfly goes through in the course of its life. You find on a tree or shrub a worm-like caterpillar. You take it in charge, placing it in a box or jar where you can provide leaves for its food and soon it either spins around itself a silken shroud, thus hiding from your sight, or else it simply seems to change to a lifeless object without eyes or wings or legs, unable to move about and motionless, save for a slight wriggle when you touch it. Yet if you keep the shroud or the mummy-like object for two or three weeks you are likely to see a beautiful moth come from the shroud or a glorious butterfly break out of the mummy case. (See plate, [page 49].) So you can get the realest kind of moving pictures by simply bringing in the caterpillars that are easily found in garden, field, and wood.
To collect these caterpillars it is only necessary to be provided with a pair of sharp eyes and an empty coffee can or some other form of tin box. Go out into the garden or along the borders of the woods. Look carefully. If you see places where leaves have been eaten, search the leaves near by and you are likely to find one or more of the caterpillars that caused the injury. Transfer them to the box and take them home with a few leaves of the food plant. There place them in some form of vivarium, which simply means a box or cage in which you can keep living creatures.
The most satisfactory cages for rearing caterpillars are those which are open above so that there is not even a glass plate between the observer and the insect. This kind of vivarium is easily made by using a band of some sticky substance like the tree tanglefoot with which trees are commonly banded, or a strip of sticky fly paper. Any wide shallow box may be used by simply placing an inch-wide band of the sticky material around the vertical sides near the top. The caterpillars will be free to move all over the open box but they cannot cross the band to escape. Fresh leaves are easily placed in the open box and the withered ones removed.
The same plan may be adopted with wide glass jars, like the ordinary battery jar. Choose a rather large one and smear the inner side near the top with a band of sticky material. The caterpillars are thus prevented from crawling out, but they are open to observation at all times. (See plate, [pages 48-49].)
In the case of the caterpillars that change to butterflies no soil need be placed in the bottom of the jar as these will attach their chrysalids to the sides or to a stick or board which may easily be put in. In the case of many caterpillars that change to moths, however, it is desirable to place about two inches of soil in the bottom of the jar. Then if the caterpillars are not cocoon spinners they can burrow into the soil when they are ready to change to pupae.
Instead of applying the sticky material directly to the glass a strip of sticky fly paper may be glued to it.
As a rule the butterfly caterpillar easiest to find lives upon cabbages. Go into the garden and you are likely to see a dozen green caterpillars upon as many cabbage plants. Bring in several of the larger ones and place them in a vivarium with some fresh cabbage leaves. In a few days some of them will be likely to fasten themselves to the vertical sides of the vivarium and shed the caterpillar skin. Each thus becomes a chrysalis. About ten days later this chrysalis skin will break open and a white Cabbage butterfly will come out.